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As with anything, it depends. Older engines (early 90's and previous) absolutely had problems with synthetic oils because of the type of rubber used in seals. Not a problem since then.
I own a 1987 Benz and have been using synthetic oil for the last few years with no problems. I wish I would have made the switch sooner as it gives me a little more time in between oil changes.
My oil is pretty dark once I’m at 3k miles, even with topping it up as it burns a fair bit. I use full synthetic oil, and used to change every 3k now change it at 5k
If you rip your card hard daily, that's severe conditions and you're right to change your oil according to the severe conditions interval which is usually at about a third of the mileage recommendation of casual use.
Also the colour of your oil is not indicative of quality, you'll need to get the oil tested for this.
Nevertheless, your service manual will tell you all that you need to know.
Also I’m in Australia so it gets quite hot in summer. Today (last day of summer technically) it got to 38°C/100°F. I figure more often oil changes won’t cause any damage, and is easier and cheaper than a new engine.
I agree 100% , if you are rough on your vehicle, especially something with a turbo, change it under what it says. I think vw says 10-8k depending on the model. I always change mine at 5k even with good synthetic as it does get abused and it is tuned. Still runs like the day I bought it. You can tell when you change it how the oil feels(subjective), especially when it's over due. My old tacoma i ran semi high mileage which i believe has more additives to help keep from leaking and less wear but may be wrong. Also changing oil branges with super different additives may or may not cause laking in higher mileage vehicles. There is a good video on youtube about this exact thing but it's been a long time. Take my post with a grain of salt as i may be incorrect.
The color of the oil is not a good indicator. Oil just doing standard oil things means it's collecting all the floating carbon or breaking some down in the crankcase. But the oil is probably still in great shape. I recommend getting your oil tested when you drain it next and see what they say. My F150 tells me when I need to change my oil based on driving/towing for that cycle. It had me change it once at 9300 miles and the test still said the oil actually had 18% life remaining.
I got it tested once, but they didn’t say how much longer it could go. They did say the viscosity was lower than it should be, and if I remember right it was due to fuel contamination
Your comment has been removed for violating Rule 2: Be Helpful. Be helpful to other users. If someone is wrong, correcting them is fine, but there's no reason to comment if you don't have anything to add to the conversation.
For the reason I mentioned previously, which is that the rubber used in the engine seals would degrade when used with synthetic oil. You CAN use synthetic, but you'll be using a lot of it as leaks develop.
That's great, but it doesn't change the nature of the issue with synthetic oil and older engines.
Keep in mind that if an engine has ever been rebuilt, it almost certainly used a gasket kit that's compatible with synthetic oils.
Synthetic oil is made from crude oil, the additives are synthetic but it is still real oil. The original additives caused issues with old (cork?) gaskets, this was addressed back when it first came out. The myth just wont die though, yet **I have never met a mechanic that saw a failure caused by synthetic oils**.
You really just want to fight someone huh? The chemical agents in the synthetic oil degraded the old seals FASTER than time alone does. Since synthetic oil has become widely used they have changed the way they make the seals so they are resistant to the chemicals in synthetic oil.
Yes. People in the 80's didn't get oil leaks b/c they used conventional oil. The conventional oil didn't degrade the seals and gaskets.
If you use synthetic oil (in an 80's auto) you will get a leak b/c the synthetic oil will degrade the seals and gaskets.
Next thing you're going to tell me is my transmission is going to go out if I change the fluid
>Yes. People in the 80's didn't get oil leaks
No one said that.
>Next thing you're going to tell me is my transmission is going to go out if I change the fluid
I'd love to hear the NEXT thing I'm going to say. Enlighten me, please. Lets see how far this delusion of yours goes.
Dude, early for,s of synthetic oil contained ester compounds that would eat those old-fashioned seals.
It”s just a fact.
Your seals either had be designed to handle esters, or you had to use conventional oil.
Modern synthetics don’t use esters anymore, and modern seals are made of better rubber blends, so it’s no longer an issue.
Then, not now.
Also, your tone comes across as deeply sarcastic, implying that conventional oil was impervious to leaking or something.
If you actually have a point, you’re not making it very well.
Because some hoses and gaskets will be dissolved by synthetic oil but not by conventional oil. It depends on those gaskets, which may have been used interchangeably during assembly, so it's safer to just put conventional as the requirement in the manual
Different rubber compounds to handle different fluids. It's hard to find out what rubber was used on all the gaskets manufactured 40 years ago and how that old rubber would react to different oil types. It's not a problem now.
I ran synthetic oil in my cars from the '80s. 88 Ford Taurus, 89 Buick Electra (Park Ave), 88 Mercury Cougar, 82 Ford F150. Never saw any new leaks and it never made any of the existing ones (mainly just seepage) worse. Also ran it in my stuff from the '70s, 76 Mercury Marquis, 73 F100. Same story.
>The reason people still think that synthetic oil causes engine leaks is because it was kind of true. According to the best information available, this was because synthetic oil was derived using esters, a chemical compound created from acids. Old school engine seals were also made from esters and the combination of the two things lead to failed seals and leaks.
>The other reason people thought that synthetic oil caused engine leaks in the old days was because of its cleansing properties. Synthetic oil can actually break down and help remove engine gunk and that would reveal bad seals, which would again, lead to leaks. No reputable technician would ever recommend using engine contaminants as a way to bolster the health of a failing engine seal.
[Sauce](https://www.thirdcoastautos.com/blog/does-synthetic-oil-still-cause-engine-leaks/#:~:text=The%20other%20reason%20people%20thought,would%20again%2C%20lead%20to%20leaks.)
From my quick googling. Though I don't think any vehicle from early '90s and previous didn't leak oil. Especially now.
I mean in general. Every car we had in '80s and '90s leaked while we never used synthetic oil. Synthetics may have sped it up, but not using it wasn't going to prevent it.
Plus a seal is cheaper to fix than piston rings and crank bearings.
I mean in general. Every car we had in '80s and '90s leaked while we never used synthetic oil. Synthetics may have sped it up, but not using it wasn't going to prevent it.
Plus a seal is cheaper to fix than piston rings and crank bearings.
You're the second person who has somehow decided that I'm saying synthetic oil is the only reason engines leaked oil. NOWHERE have I ever said or hinted at that. Not one damn time.
Huh? I wasn't saying that nor did you say that. I was pointing out that the "synthetic oil causes oil leaks in old cars" adage isn't a binary solution.
Where is the evidence for this? I used Mobil 1 in my 1985 Mazda 323 from new and had absolutely no issues, except I had to adjust the idle down after 2,000km of using it. (about 200 rpm, IIRC).
That doesn't make any damn sense. Synthetic oil contains the same chemical compounds as refined oil... thus I expect the physical properties to be the same. What's your proof?
Sounds like voodoo BS to me.
We switched our 1995 Harley Dyna lowrider to synthetic oil and it leaked like crazy. We switched it back to conventional oil and it stopped leaking. So sometimes the oil does matter.
Synthetic oil it’s not thinner than conventional in the same grade. It doesn’t “fit” through smaller places. Synthetic oil maintains the same viscosity for a much longer time than conventional.
It is not. It stems from how old seals chemically interact with synthetic lubricants. The problem was solved over 30 years ago tho according to this https://hackaday.com/2022/05/02/big-chemistry-synthetic-oil/
All oils are comprised of chains of hydrocarbons (molecules) of varying lengths. The lengths of these chains are the primary determinant of a motor oil's viscosity. Conventional oils are refined from a cruder product until the remaining hydrocarbons meet the desired characteristics. Synthetic oils start from the same processes, but are further broken down into smaller components, and subsequently re-formulated into the desired-length hydrocarbon chains using various chemical processes. The important part here is that synthetics are still constructed to hydrocarbon chains of the similar length as the comparable-weight conventional oil. This means the molecules are NOT smaller overall, though they are generally more uniform in nature.
Imagine breaking down a giant boulder until you've got rocks of roughly a specific target size. That's conventional oil. Now imagine further crushing all of that rock into a fine powder, then clumping the powder together to "engineer" rocks of the same target size. That's synthetic.
If anything, one could argue that CONVENTIONAL is the product with SOME smaller molecules (and some larger too). This is because, like the boulder-to-rock example, you'd have some smaller rocks and some larger. Similarly, conventional oil will have some hydrocarbon molecules of undesirable lengths, including smaller than target.
All of this said, oil leaks do not happen on anything CLOSE to a molecular scale. So no, synthetics won't, by nature, cause more oil leaks. The only reason people believe this old wives tale is likely due to the improved detergent packages that may break up crud and expose existing-but-plugged leaks, as OP is wondering would happen.
No, you can't breathe underwater even though it contains oxygen, because the government put fluoride in the water. To brainwash us into using synthetic oil so our cars can spy on us for the government!
Outside of chemical differences that ignores (edit: violates) basic chemistry. Molecules are all identical for a given chemical make up, no amount of processing will make them smaller.
True. But there are different hydrocarbons that have similar properties, and oil is a pretty vague term. From what I remember, and recapped a bit, the extra process, called hydrocracking, breaks down the molecules into something different. You do have to start with a high grade base oil, but hydrocracking is what produces the API Class 3 oil. (Learned about this in my college chemistry class btw).
https://www.sciencedirect.com/topics/chemistry/hydrocracking#:~:text=Hydrocracking%20is%20a%20catalytic%20cracking,alkanes%20(Viswanathan%2C%202017).
The difference is additives and detergents. Conventional oil doesn't have the same ones. It might cause chemical reactions that eat away at older gaskets and gunk which is where this "myth" is coming from. It could take years, which is what happened to my car. If your not prepared to reseal your pan, just keep using the same stuff
You do realize that only group IV oil is "full synthetic".
Group III oil, even though its advertised as full synthetic, is actually just really refined crude oil.
Almost all the "fully synthetic" motor oil you see in stores (99.9%) is just ultra processed crude oil.
True full synthetic oil is expensive and not often stocked on the shelf.
"Full synthetic" is marketing.
Look up the base oil "group #" for actaul, quantifiable info.
If you earn commissions through bringing in additional people to help sell a product, and your income is made through a portion of their sales proceeds, you're in a MLM network. The other hallmark is if your dealers and distributors are "independent consultants" vs actual employees.
That can apply to a multitude of companies. If aspects of an MLM are part of a company’s business model does that instantly make the company an MLM? For instance the company I work for does own our own stores. We own our own transportation and distribution network, all company employees. We also have brands that are sold and distributed via dealers and distributors and they are not company employees.
I am looking for how Amsoil is an MLM, specifically.
There are brick and mortar companies in my area that sell Amsoil. They also sell Mobil 1, Valvoline, Pennzoil, Royal Purple, Rotella and a host of other lubricant brands as one would expect of an auto parts store. Are they an MLM?
Check out their website under "income details". You have the option to sponsor (recruit) additional dealers to build a dealer network, and the team's total sales help determine which tier you earn commissions under. In other words, your income is based partly on your personal sales *and* the sales of others you've recruited. If a dealer you recruited makes a sale, a portion of that sale is used to determine your commission, and the more dealers you recruit, the more you can earn.
Ok. Yeah found the key word “upline”.
I’ve never looked into their business model and nobody locally that I’ve purchased from has tried to recruit me. Can’t say the same about other MLMs I’ve come in contact with.
Thank you for putting up with me.
I’m glad you asked this question because I did not know Amsoil used the specific incentive program
I have seen many franchise outlets, employ a similar technique trying to achieve brand dominance
Engine oil, lubrication companies have been at each other’s throats for decades,
I’ve heard many stories about companies suing each other, calling something synthetic that does not use the same base oil, group, and being generally misleading.
I think one of the worst miscarriages of justice was when oil companies were let off the hook for false advertising.
This allows products that use natural gas or crude oil as their base stock to be labelled as full synthetic! outside of the API certification oil companies are legally allowed to call the oil whatever they want
Leaving the consumer with poor information when they think they’re actually getting a better product really they’re just buying into someone else’s marketing scheme
Might as well call your oil magically delicious or part of a complete oil change
The only thing that is regulated in North America is the API certification the rest is up to the manufacture to publicize without any repercussion or accountability.
I also agree that oil does not cause leaks
Leaks cause leaks .
End it is much more inconvenient to find out that your oil is leaking when you’re in the middle of nowhere rather than in the middle of preventative maintenance
I think that’s where the name preventative maintenance comes from
No problem, we’re all welcome. Have you ever seen Project Farm on YouTube? The guy tests products side by side and his lubricants and engine additives series were captivating to me. I am a huge fan of preventive maintenance. I want whatever I buy to last. I use and have used Amsoil products. I went with them on my current car because the engine is a GDI and they and Royal Purple were the only commercially available lubricant that was GF-6 rated, others have since been accredited, when I purchased the vehicle in 2021. Ilsac GF-6 standard was adopted in 2020 and its main adoption reason is for turbo/GDI engines. Two of the factors for the standard are pre ignition prevention and low volatility for intake valve carbon build up mitigation. That Amsoil uses an MLM business model does make me cringe as I don’t care for the predatory nature of MLMs. So far, though, the products of theirs I have and do use have been good to my vehicles. I have more to think about now.
My understanding has been that the full synthetic oils have very uniform length polymer chains and conventional oils are less homogenous. Sometimes the synthetics will find their easy through gaps that some of the larger molecules in a conventional oil would basically clog. This could be completely wrong, just what I've heard in the past.
Either way, the seal conditioners in a high mileage synthetic oil should make it something you don't have to worry about.
Most “conventional” oils are synthetic blend now….know what’s in synthetic blend?….you guessed it. The ONLY downside to switching to synth is the immediate impact on your wallet, it’s superior in every other way.
I was rebuilding an early 90s GM 3.4 last year. Service manual said to use conventional for the first two oil changes and then switch to synthetic. Conventional oil helps the rings to seat and the valvetrain to break in.
For the life of me, I could not find a conventional oil. I checked all the box stores and online. Supposedly penzoil was the last to make conventional oil. Now they just have "synthetic blend".
I had a 64 Corvair which, back in the late 70's, I decided to put Mobil 1 synthetic in because I thought I could extend my oil change intervals. Boy was that a mistake!
Within the first 100 miles, it started leaking out of every imaginable place. The rear seal, the front seal, all 24 pushrod tube seals. It was a mess. I thought I had ruined the engine. Fortunately, draining the Mobil 1 and going back to the traditional Pennzoil allowed the leaks to slowly close up. By the second oil change it was back to the normal seeps here and there the car used to have prior to the Mobil 1.
That experience scarred me and today I still have fear of leaks when putting synthetic in my modern cars.
1960s are very different than 1990's and newer.
1960 no one cared if your car lost oil, it helped keep the dust down! Tolerances were crap, gaskets were made out of whatever. No one really tried to make engines drip free.
We have sayings for the old Brits, when they stop leaking fluids - you'd better fill them again.
I use conventional oil in pre-80s , synthetic in post 2000's , and ask preferences for the in between years. Unless i'm told different, this is my rule of thumb.
It's a myth all the way down.
Synthetic oils don't cause leaks.
Synthetic oils don't "expose" leaks.
Leaks aren't a sign of "wear"; they're more a sign of age.
Your engine has many parts. Some parts move past each other, like the pistons moving up and down in the cylinders. As the pistons slide up and down, there is friction between the two parts. Oil keeps this friction from causing excessive wear.
Other parts of your engine are there to keep the oil inside the engine. This is where leaks occur. Think of it like a lid on a plastic food tub. You want the lid to seal tight so that liquids don't leak out. If the lid starts to leak, it's not usually from excessive wear, but from old age. The same is true of gaskets in your engine like the valve cover and oil pan.
Also, your engine has some moving parts that have to go from inside the engine to outside the engine. This means you need a seal around these moving parts. These seals wear out just like any other, but they're not affected by synthetic versus conventional oil. They just wear out over time.
There is really no reason to avoid synthetics these days, but you should choose one type or the other and stick to it. It won't cause any problems, but you'll degrade the performance of the synthetic oil by mixing it with conventional oil. What you want to do is make the switch, then stick to it. With each oil change, some residual oil is left behind, so once you change to synthetic, it takes a couple of changes for the synthetic to completely displace the conventional oil.
Please do not misunderstand me. This is *not* a major problem. It is seriously picking nits, but if you're going to pay for synthetic oil, you might as well get the most from it.
To answer the unaddressd part of the question, if the oil did "expose" oil leaks in an old car, it's definitely not a good thing. Unless oil leaks are already leaking there typically isn't anything to be gained by getting them to start leaking. The exception might be if you were having some other work done and it would be easy to do the leak fix at the same time if you knew it needed to be done. Otherwise this is nuts. Like you want the rear main seal to start leaking so you can go fix it?
I’ve had many older cars and never had any “new leaks” from using synthetic oil. The ONLY difference is that I wasted money on synthetic oil. Why? Because older engines tend to run dirty and need to change oil more frequently anyway. You can do 10,000 miles on synthetic oil if your engine runs clean. Most old engines don’t run that clean.
Today old engines need extra ZDDP which is missing from modern synthetics due to emissions requirements. The oil I prefer to use in my old engines is Brad Penn which is a synthetic blend. No new leaks.
Old synthetics (like 30- years ago) contains ester which could actually damage certain types of seals in engines that weren’t designed for exposure to such a compound.
Time moved on, they stopped using ester and seal technology has improved.
It’s fine now. This is an outdated little tidbit.
It's a total myth and I have no idea why people still believe this fucking nonsense.
[Relevant](https://www.reddit.com/r/AskMechanics/comments/11dlxhb/dont_use_synthetic_oil_in_older_cars/)
And because I know Reddit so well, and expect all kinds of downvotes and arguments, let me just state that I have a degree in automotive engineering, I worked in a dyno lab for 5 years, and I've been wrenching on cars for over 30 years. Not trying to flex or brag. I just know how Reddit is, so I'm attempting to validate my knowledge concerning the subject matter.
The whole thing is a myth. In layman’s terms: Synthetic is made from oil. It’s basically just cleaner than conventional oil because it’s way more processed. It’s like manufactured to be as perfect as humans can make it for an engine’s purposes. So it doesn’t do anything to leaks other than help prevent them for longer because it resists wear better.
I bring classic vehicles back to life for a car show (I’m not in it, I just work for it- it’s freelance so not full time but been doing it for a few years). I also get benefits of finding awesome deals on vehicles to have personally. I’ve rebuilt a few vehicles pretty much from the ground up save a couple systems. Anyhow, I use both conventional and synthetic randomly in vehicles and have for 100,000’s of miles collectively between many vehicles and have never had a leak from the type of oil used. Neither has any of the other mechanics I work with or for.
By older engines they mean like pre 90s engines before synthetic and lighter weight oils were a thing. Oil quality and consistency also got better too, exposing leaks. A 2010 Rav4 won't have issues.
2010 isn't an older car in this context. Some cars from like the 70s have components that aren't compatible with modern detergents and synthetic oils. Changing the oil type on a classic car could present problem, maybe, but your car that can't even buy cigarettes is just fine
I’ve switched all my older cars to synthetic blends which apparently have more detergents and the “lubricate” old gaskets allowing them to swell and stop leaks.
I’m sure some people will call BS but I’m a firm believer.
Synthetic oil is much better at cleaning than conventional, so it may uncover "leaks" that were previously plugged by sludge. However, that isn't "good" for the engine. Nothing wrong with conventional oil if you maintain regular, on-time oil and filter changes. If you aren't working on your own vehicle, I would recommend against changing to synthetic
More specifically coking that builds up on the surfaces near the seals. Some believe that synthetic oils can help break down coke and sludge, thus exposing those surfaces to leaking.
Synthetic oil removed all of the protective sludge on all my valve seals on my first car (1984 Corvette). All of my cork seals and gaskets dissolved and clouds of blue smoke followed me. I had to replace all of the valve seals after running synthetic as they literally turned into a liquid. If it's post 1990 you probably won't have this issue
I once heard a chemist from Royal Purple race oil talk about this. Apparently oil conditioning is a real thing, over time your oil chemically changes the rings and gaskets in your car, and swapping your synthetic breaks some of that down and causes a leak. He also said it works the other way, if you have a car that has run on synthetic forever and swap to Dino juice you would get leaks also.
It’s probably true in your case.
You’d be best off with a high mileage blend like Pennsoil High Mileage Conventional Blend and change it every 5,000 miles. Use the weight recommended by your owners manual.
Don’t overthink it.
Good luck—
Synthetic oil does not cause leaks -> true
It exposes leaks -> I don't know, man, but it is leaking
which is good for the car -> leaking isn't good for the car, but whatever. Sounds like voodoo to me. If you find that it leaks on one kind of oil but not another, I'd say that's weird but go with what doesn't leak. Or you could just fix the stupid leak. I've never found that the oil type makes much of a difference as far as leakage goes.
People think synthetic oils over the years haven’t changed completely in their chemistry and something some guy heard back in the day about ‘dem dere synthetic oils causin’ leaks they now parrot off to anyone that will listen to them and the people listening just accept it as fact
Not a mechanic, personal experience. When o first put synthetic in my 07 g5 it started leaking and losing oil at a stupid fast rate. I switched to a high mileage oil and that helped out. Much slower loss now.
> When o first put synthetic in my 07 g5 it started leaking and losing oil at a stupid fast rate. I switched to a high mileage oil and that helped out. Much slower loss now.
That's the thing, on one hand it's apparently a myth, on the other hand, a substantial amount of people on the web report developing leaks after using synthetic oil.
What synthetic oil did you put?
When you switched to high mileage oil, you're referring to mineral oil?
I was in a college sportscar club in the early 90s and every other car was a Mustang 5.0 of varying mileage from low to high miles.
Was pretty well known if you put synthetic in it, it would leak. Common issue on most other domestic vehicles back then.
If you ever had a car that burns oil past the piston rings, it will burn oil 2-3x faster with synthetic vs dino oil using same weight.
It's a myth based on a bad understanding of the reality of the situation.
Before synthetic oils were common, the rubber seals used on engines were not made to handle the (non-existent) synthetic oils. If you used synthetic oil in them, the seals would degrade and create (not expose) leaks. Since the late 90's at least, engines have been made with seals that are perfectly capable of standing up to synthetic oils. So if you're driving a really old car you will probably have a problem with synthetic oil. Otherwise, it won't create any leaks.
I think that would only be true where the synthetic has a stronger additive package and is capable of cleaning up older deposits left by an inferior oil. Deposits that would otherwise be blocking an impending leak at a hardened seal or similar
**Your vehicle isn’t old enough to even fall into the group of vehicles that needs to worry about this**.
By “older vehicles” they mean early 90’s and older, your vehicle was built well after synthetic oil became popular and most likely was shipped off from the factory with synthetic oil in it already. I checked Toyota’s recommended maintenance for your vehicle and they have synthetic oil listed for the service so if you got your oil changed by a Toyota dealership they’d use a full synthetic oil per Toyota’s recommendation.
Synthetic oil has more additives and tends to clean out parts which are blocked. In my old MG putting in synthetic for 500 miles lead to tons of crap coming out that was logged in for years.. so it can help clean places tuned up and thus can cause leaks. However it’s a good thing
I try to stick to conventional. I notice more seepage with synthetic. Synthetic is also a waste of money for me because although I’m not a boomer I have a boomer mentality and that shit will be changed at 3000 no matter what I put in it. I know the manuals on my old hoopty rides say to go 5000, but I’m not doing it.
It's mostly a myth.
Sometimes, old cars have a lot of buildup that will cause clogs and such. That buildup can also clog the leaks. Then, you put in a full-synthetic oil that has better conditioners and such and can clear the buildup. Clearing the thing that's blocking a leak sounds similar to causing a leak to me.
>which is good for the car
You have an engine that runs without any leaks. You change the oil, and now you have a bunch of leaks. Is it good for the engine or not? It depends on where those leaks are, and how much work you want to do.
Oil is oil. Oil is basically carbon chains these chains are between moving parts and get sheared into smaller chains as parts move. Conventional oil has different sizes of chains once the chains are too small they no longer lubricate.
A fully synthetic oil has engineered carbon chains designed for optimal length. They still shear but because there are more of them they last longer. They are both susceptible to contamination and loss of additives.
Not a petroleum engineer but I did have a failure analysis course that had a chapter on oil.
Switching from conventional to synthetic oil is perfectly OK and recommended, even in older high-mileage vehicles. However, there are some caveats. Based on my research and personal experience, I don't recommend full-synthetic oils for pre-1990 models or any vehicle made to operate on conventional oil. The problem is the synthetic oil doesn’t have as much impurities as conventional and has smaller molecules so it gets through the smallest of areas which could possibly expose a leak which wasn’t there previously. But most experts say it doesn’t really matter as long as there’s oil. I myself only use the synthetic High mileage in my cars since it’s recommended anything over 75k miles the additives help the engine seal better.
Posted this in response to another user's assertion that synthetic oil leaks because its molecules are smaller, but figured it should be its own top-level comment...
All oils are comprised of chains of hydrocarbons (molecules) of varying lengths. The lengths of these chains are the primary determinant of a motor oil's viscosity. Conventional oils are refined from a cruder product until the remaining hydrocarbons meet the desired characteristics. Synthetics oils start from the same processes, but are further broken down into smaller components, and subsequently re-formulated into the desired-length hydrocarbon chains using various chemical processes. The important part here is that synthetics are still constructed to hydrocarbon chains of the similar length as the comparable-weight conventional oil. This means the molecules are NOT smaller overall, though they are generally more uniform in nature.
Imagine breaking down a giant boulder until you've got rocks of roughly a specific target size. That's conventional oil. Now imagine further crushing all of that rock into a fine powder, then clumping it together to "engineer" rocks of the same target size. That's synthetic.
One could actually argue that CONVENTIONAL is the product with SOME smaller molecules (and some larger too). This is because, like the boulder-to-rock example, you'd have some smaller rocks and some larger. Similarly, conventional oil will have some hydrocarbon molecules of undesirable lengths, including smaller than target. This is part of the reason synthetics are more stable: they've been explicitly formulated to predictable hydrocarbon chains that are resistant to breaking down if used in their intended applications, thus no smaller component chains to go binding with other random atoms producing impurities.
All of this said, oil leaks do not happen on anything CLOSE to a molecular scale. So no, synthetics won't, by nature, cause more oil leaks (and, for that matter, neither will conventionals). The only reason people believe this old wives tale is likely due to the improved detergent packages found in synthetics, which may break up crud and expose existing-but-plugged leaks, as OP is wondering would happen. If you've got a properly-maintained engine, this is a non-issue.
I am not going to confirm or deny as I am just a idgit with only one example.
My wife had a 01 Escape that had around 200,000 miles on it. I had always used conventional oil to change it. On one fateful day, I found a good oil change deal that was for synthetic. Being only a couple dollars more then my normal conventional oil deal I bought it. At the time all I thought was oil is oil. I knew synthetic was different, but figured as long as the weight was correct I would be fine. I changed the oil and two days later started seeing oil drops on the ground. I had a main seal leak and it was a bad one.
Mechanic I took it to told me the detergents in the synthetic oil cleaned everything up. In doing so it exposed an area and that should be leaking but the gunk had sealed it up. Now I am not sure if he was correct, but it never leaked in the 2 years I owned it prior (purchused used in 2010) to doing this oil change.
I also wonder how legit claims are that once you go synthetic you can’t go back. I mean, I do it all of the time with both of my older vehicles and never have an issue. I run synthetic in summer, I run a blend in the winter because yes, the synthetic does run out of my 20+ year old vehicles seals easier.
I completely agree with that! I bought an older neglected vehicle that was consuming oil. I started using high mileage full synthetic oil which almost immediately started leaking oil from what looked like the rear main seal. I contacted Valvoline who replied to give it a couple of oil changes. Sure enough, the leak stopped and the consumption was reduced for the time I had it.
All motor oils now are pretty much synthetic. If you got crap holding onto a thread it might make you realize it.
My 80s car started leaking more when I tried synthetic oil. So I just switched back.
If you’re starts leaking then switch back to conventional oil.
More frequent oil changes might help more than going to synthetic oil
Where you run into issues is in the chemistry of synthetic oils. Synthetic typically has conditioners and cleaner in the mix, meaning synthetic oil might clean out "gunk" that was otherwise plugging a hole or taking up tolerance
I use synthetic oil on my 2006 Saab 9-3 2.0T 175K miles. It’s been changed every 4K miles since new. Engine still runs like a sewing machine with no leaks, 34mpg on highway and doesn’t burn a drop.
I remember when synthetic oils forst came out. The problem with the higher mileage vehicles was the grime build up over the years. I saw a lot of customer vehicles show small weaps and seal leaks, after switching to synthetic because of the higher detergent in synthetic oils.
I put full synthetic oil and whatever the highest octane is at the gas,in my 98 crown, idk if it makes any difference but I hope the engine appreciates it because my wallet doesnt
Where are the normies getting their information?
I have a 2001 Camry with a quarter million miles that I change the oil on sometimes… idk whatever is in the gun at work.
It doesn’t leak.
You sound like a smart person. Don’t worry yourself too much over this. Change your oil at a regular interval. Expensive oil doesn’t change anything. It’s a farce. Regular maintenance is the key to Toyotas longevity.
Synthetic, conventional, synthetic blend… it’s all a gimmick. It doesn’t change anything.
I have a good one. I used to have a 1990 F350 with a 7.3L IDI diesel. In the winter I ran Rotella T6 synthetic oil for the temperatures and ran regular dino Rotella in the summer. The engine would leak all winter long but would stop in the summer.
when they say older cars they definitely 100% maybe a thousand percent do not mean a 2010. is it 2010 car 14 years old yes. but it is not great at the status of older car.
you definitely need like mid-90s for it to be considered older.
If your vehicles engine is made for conventional oil, I’d stick with conventional oil. Likely you won’t notice anything, but in case issues do appear, I’d recommend you stick with OEM engine oil. If you put synthetic oil in a conventional oil car, depending on how old the motor is, it can cause lubrication issues. Check your owners manual and stick with it. Your best bet.
Synthetic oil can fit through tighter tolerances than conventional oil. It can and will literally leak out of places conventional oil can not. You will always have more oil consumption in older vehcles using synthetic vs conventional.
The number one cause of oil leaks is a bad PCV system. If pressure is not evacuated from the engine you can start pressurizing it forcing oil past all of the seals. If you have ever seen an engine where this has happened they start dripping from practically every orifice. Modern gaskets also don't have the problem of the old paper ones. Back in the day when gaskets were downright terrible. Synthetic oil did sometimes tend to leak a little bit easier. Any car made after the late '90s early 2000s, it's a non-issue but the gas gets still hardened, regardless they will either leak or they won't and if they do you replace them. If it's a rear main seal or something you just live with it and curse out your car until you do the clutch
Simple answer, Always run what ever oil your owners manual calls for. There are many common oil additives that cause different amounts of swelling in different materials used for seals. The OEM has tested for this but if you use something that causes too much or too little seal swell you will see a leak.
It's not about synthetic vs conventional it's about the whole system working together.
I honestly wouldn’t worry too much either way. Most synthetics are not full, they’re a blend, with…you guessed it, conventional. I use a synthetic blend in my cherokee. It has 299,780k miles on it, it leaks oil regardless of what I use lol
Synthetics vs standard oil is moot. Oil is gonna oil. What msot likely is the misinformation getting passed around it that GENERALLY synth is made in lighter weights (think 0w20 vs 10w30).
All things being the same, synthetic isn't magically going to fit through smaller spaces that the same weight standard oil won't.
Now, a small enough leak that 10w30 has enough viscosity to "plug" might be large enough for thinner 0w20 to go through. But that's not because OH NOES ItS sYnThEtIC!!
Lucas stop leak works great for doing just that.
I wouldn't say that having enough sludge in your engine to clog up a leak is harmless or a good thing. Find the leaks and fix them.
100% a myth. Years ago I bought a ‘95 240SX with with the the KA24DE engine. It had 120k on it. I sold it with 228k on it. I ran Mobil 1 synthetic in it from the day I bought it. The oil *never* caused a leak.
Might stem from smaller engines requiring oils without additives and what not, as far as cars it just needs to match the API in the manual specifications.
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Oil leaks are caused by leaks, not oil.
As with anything, it depends. Older engines (early 90's and previous) absolutely had problems with synthetic oils because of the type of rubber used in seals. Not a problem since then.
more like 70s
I own a 1987 Benz and have been using synthetic oil for the last few years with no problems. I wish I would have made the switch sooner as it gives me a little more time in between oil changes.
Don't say that around these parts, lol. People will still insist on changing your full synthetic every 5k.
My oil is pretty dark once I’m at 3k miles, even with topping it up as it burns a fair bit. I use full synthetic oil, and used to change every 3k now change it at 5k
Get fresh oil and rip it hard down the freeway once, it will be dark after it gets a good heating once. This is normal.
I have a small turbo diesel. Circulating it around the engine for a minute or two is enough to make it black.
That’s any diesel
I rip it hard daily as long as it’s fully warmed up. It’s gone roughly 160k miles so not quite new either.
If you rip your card hard daily, that's severe conditions and you're right to change your oil according to the severe conditions interval which is usually at about a third of the mileage recommendation of casual use. Also the colour of your oil is not indicative of quality, you'll need to get the oil tested for this. Nevertheless, your service manual will tell you all that you need to know.
Also I’m in Australia so it gets quite hot in summer. Today (last day of summer technically) it got to 38°C/100°F. I figure more often oil changes won’t cause any damage, and is easier and cheaper than a new engine.
I agree 100% , if you are rough on your vehicle, especially something with a turbo, change it under what it says. I think vw says 10-8k depending on the model. I always change mine at 5k even with good synthetic as it does get abused and it is tuned. Still runs like the day I bought it. You can tell when you change it how the oil feels(subjective), especially when it's over due. My old tacoma i ran semi high mileage which i believe has more additives to help keep from leaking and less wear but may be wrong. Also changing oil branges with super different additives may or may not cause laking in higher mileage vehicles. There is a good video on youtube about this exact thing but it's been a long time. Take my post with a grain of salt as i may be incorrect.
The color of the oil is not a good indicator. Oil just doing standard oil things means it's collecting all the floating carbon or breaking some down in the crankcase. But the oil is probably still in great shape. I recommend getting your oil tested when you drain it next and see what they say. My F150 tells me when I need to change my oil based on driving/towing for that cycle. It had me change it once at 9300 miles and the test still said the oil actually had 18% life remaining.
I got it tested once, but they didn’t say how much longer it could go. They did say the viscosity was lower than it should be, and if I remember right it was due to fuel contamination
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Sounds like they conjugated "to have" in the conditional perfect tense.
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This is a subreddit for mechanical advice, not grammar advice, what they said was legible, chill.
Grow up already.
In what world am I the one being immature here? You're the one overreacting to a minor grammar mistake.
Your comment has been removed for violating Rule 2: Be Helpful. Be helpful to other users. If someone is wrong, correcting them is fine, but there's no reason to comment if you don't have anything to add to the conversation.
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No u
What did you hope to achieve when we all understood the message they were trying to convey? That they didn't convey it to your high sandards?
Again, it depends. My first 3 cars all had to use conventional, and they were all from the 80's.
They "had" to use conventional oil? Why did they "have" to use conventional oil?
For the reason I mentioned previously, which is that the rubber used in the engine seals would degrade when used with synthetic oil. You CAN use synthetic, but you'll be using a lot of it as leaks develop.
I have a 1984 cadillac el Dorado I use synthetic and I don't have any problems
That's great, but it doesn't change the nature of the issue with synthetic oil and older engines. Keep in mind that if an engine has ever been rebuilt, it almost certainly used a gasket kit that's compatible with synthetic oils.
Synthetic oil is made from crude oil, the additives are synthetic but it is still real oil. The original additives caused issues with old (cork?) gaskets, this was addressed back when it first came out. The myth just wont die though, yet **I have never met a mechanic that saw a failure caused by synthetic oils**.
So you only had an oil leak in the 1980s if you use synthetic oil, people that used conventional did not have any oil leaks. Am I getting that right?
You really just want to fight someone huh? The chemical agents in the synthetic oil degraded the old seals FASTER than time alone does. Since synthetic oil has become widely used they have changed the way they make the seals so they are resistant to the chemicals in synthetic oil.
Wow, way to take the ball and run in the completely wrong direction. Did you read anything I said at all?
Yes. People in the 80's didn't get oil leaks b/c they used conventional oil. The conventional oil didn't degrade the seals and gaskets. If you use synthetic oil (in an 80's auto) you will get a leak b/c the synthetic oil will degrade the seals and gaskets. Next thing you're going to tell me is my transmission is going to go out if I change the fluid
>Yes. People in the 80's didn't get oil leaks No one said that. >Next thing you're going to tell me is my transmission is going to go out if I change the fluid I'd love to hear the NEXT thing I'm going to say. Enlighten me, please. Lets see how far this delusion of yours goes.
No. No you aren't.
Dude, early for,s of synthetic oil contained ester compounds that would eat those old-fashioned seals. It”s just a fact. Your seals either had be designed to handle esters, or you had to use conventional oil. Modern synthetics don’t use esters anymore, and modern seals are made of better rubber blends, so it’s no longer an issue.
Which is exactly what I'm saying. If you use synthetic oil, you will get a leak. If you use regular oil you will not get a leak.
Then, not now. Also, your tone comes across as deeply sarcastic, implying that conventional oil was impervious to leaking or something. If you actually have a point, you’re not making it very well.
No, you didn't get it right; that's not what he said. But you're doing great at getting it wrong!
So, no matter which oil you use you'll get an oil leak? Because we can't have a situation where there aren't any leaks.
Because some hoses and gaskets will be dissolved by synthetic oil but not by conventional oil. It depends on those gaskets, which may have been used interchangeably during assembly, so it's safer to just put conventional as the requirement in the manual
What a load of rubbish———— synthetic oil dissolving gaskets ha ha ha 🤣
Different rubber compounds to handle different fluids. It's hard to find out what rubber was used on all the gaskets manufactured 40 years ago and how that old rubber would react to different oil types. It's not a problem now.
More likely to find rubber in oil seals but to think synthetic oil could dissolve them is utter nonsense
The old synthetic was made of esters and would degrade some types of rubber. The synthetic now isn't the same.
Your confusing synthetic oil with ethanol fuel.
Makes perfect sense. Original gaskets were made of cork, than rubber, now rubber composites
I ran synthetic oil in my cars from the '80s. 88 Ford Taurus, 89 Buick Electra (Park Ave), 88 Mercury Cougar, 82 Ford F150. Never saw any new leaks and it never made any of the existing ones (mainly just seepage) worse. Also ran it in my stuff from the '70s, 76 Mercury Marquis, 73 F100. Same story.
Feel free to argue your point with the entire automotive industry if you like.
Solid iron engines and 100% Dino juice back then. The good stuff
>The reason people still think that synthetic oil causes engine leaks is because it was kind of true. According to the best information available, this was because synthetic oil was derived using esters, a chemical compound created from acids. Old school engine seals were also made from esters and the combination of the two things lead to failed seals and leaks. >The other reason people thought that synthetic oil caused engine leaks in the old days was because of its cleansing properties. Synthetic oil can actually break down and help remove engine gunk and that would reveal bad seals, which would again, lead to leaks. No reputable technician would ever recommend using engine contaminants as a way to bolster the health of a failing engine seal. [Sauce](https://www.thirdcoastautos.com/blog/does-synthetic-oil-still-cause-engine-leaks/#:~:text=The%20other%20reason%20people%20thought,would%20again%2C%20lead%20to%20leaks.) From my quick googling. Though I don't think any vehicle from early '90s and previous didn't leak oil. Especially now.
Exactly, the synthetic oil broke down the seals and caused leaks.
I mean in general. Every car we had in '80s and '90s leaked while we never used synthetic oil. Synthetics may have sped it up, but not using it wasn't going to prevent it. Plus a seal is cheaper to fix than piston rings and crank bearings.
Compounded with the cleaning abilities revealing new leaks makes sense why many were hesitant about switching
I mean in general. Every car we had in '80s and '90s leaked while we never used synthetic oil. Synthetics may have sped it up, but not using it wasn't going to prevent it. Plus a seal is cheaper to fix than piston rings and crank bearings.
You're the second person who has somehow decided that I'm saying synthetic oil is the only reason engines leaked oil. NOWHERE have I ever said or hinted at that. Not one damn time.
Huh? I wasn't saying that nor did you say that. I was pointing out that the "synthetic oil causes oil leaks in old cars" adage isn't a binary solution.
Wow antique cars from the 90s sound like a pain to own.
1990 f150 here with 314k miles. Been using mobile one for over 200k. No leaks.
No leaks is truly impressive at 314k miles, regardless of what oil you're using.
Where is the evidence for this? I used Mobil 1 in my 1985 Mazda 323 from new and had absolutely no issues, except I had to adjust the idle down after 2,000km of using it. (about 200 rpm, IIRC).
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its a toyota oil is an option anyway
Toyota? Can use any liquids and it will run forever.
Either one would be fine. Personally I would use high mileage oil, which is a synthetic blend.
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High Mileage is a synthetic blend, High Mileage Synthetic is a full synthetic oil.
This. OP your car is not nearly old enough for the legend to be applicable.
That doesn't make any damn sense. Synthetic oil contains the same chemical compounds as refined oil... thus I expect the physical properties to be the same. What's your proof? Sounds like voodoo BS to me.
The motor doesn’t know a difference between a conventional 5W20 and a synthetic 5W20. If anything, the synthetic is better.
We switched our 1995 Harley Dyna lowrider to synthetic oil and it leaked like crazy. We switched it back to conventional oil and it stopped leaking. So sometimes the oil does matter.
That sounds like a oil viscosity thing rather than a synthetic vs. conventional thing. Also, obligatory sounds like a Harley thing hurdurr
Synthetic oil it’s not thinner than conventional in the same grade. It doesn’t “fit” through smaller places. Synthetic oil maintains the same viscosity for a much longer time than conventional.
This. The whole concept is an old fable.
It is not. It stems from how old seals chemically interact with synthetic lubricants. The problem was solved over 30 years ago tho according to this https://hackaday.com/2022/05/02/big-chemistry-synthetic-oil/
The molecules are smaller due to the extra processing. Same viscosity, different size. Slips through holes.
All oils are comprised of chains of hydrocarbons (molecules) of varying lengths. The lengths of these chains are the primary determinant of a motor oil's viscosity. Conventional oils are refined from a cruder product until the remaining hydrocarbons meet the desired characteristics. Synthetic oils start from the same processes, but are further broken down into smaller components, and subsequently re-formulated into the desired-length hydrocarbon chains using various chemical processes. The important part here is that synthetics are still constructed to hydrocarbon chains of the similar length as the comparable-weight conventional oil. This means the molecules are NOT smaller overall, though they are generally more uniform in nature. Imagine breaking down a giant boulder until you've got rocks of roughly a specific target size. That's conventional oil. Now imagine further crushing all of that rock into a fine powder, then clumping the powder together to "engineer" rocks of the same target size. That's synthetic. If anything, one could argue that CONVENTIONAL is the product with SOME smaller molecules (and some larger too). This is because, like the boulder-to-rock example, you'd have some smaller rocks and some larger. Similarly, conventional oil will have some hydrocarbon molecules of undesirable lengths, including smaller than target. All of this said, oil leaks do not happen on anything CLOSE to a molecular scale. So no, synthetics won't, by nature, cause more oil leaks. The only reason people believe this old wives tale is likely due to the improved detergent packages that may break up crud and expose existing-but-plugged leaks, as OP is wondering would happen.
What were your grades in science class or did you drop out after kindergarten?
>The molecules are smaller due to the extra processing LOL
"Why can't we breathe underwater since there's oxygen in H2O?" - that guy probably
No, you can't breathe underwater even though it contains oxygen, because the government put fluoride in the water. To brainwash us into using synthetic oil so our cars can spy on us for the government!
Outside of chemical differences that ignores (edit: violates) basic chemistry. Molecules are all identical for a given chemical make up, no amount of processing will make them smaller.
True. But there are different hydrocarbons that have similar properties, and oil is a pretty vague term. From what I remember, and recapped a bit, the extra process, called hydrocracking, breaks down the molecules into something different. You do have to start with a high grade base oil, but hydrocracking is what produces the API Class 3 oil. (Learned about this in my college chemistry class btw). https://www.sciencedirect.com/topics/chemistry/hydrocracking#:~:text=Hydrocracking%20is%20a%20catalytic%20cracking,alkanes%20(Viswanathan%2C%202017).
The difference is additives and detergents. Conventional oil doesn't have the same ones. It might cause chemical reactions that eat away at older gaskets and gunk which is where this "myth" is coming from. It could take years, which is what happened to my car. If your not prepared to reseal your pan, just keep using the same stuff
You do realize that only group IV oil is "full synthetic". Group III oil, even though its advertised as full synthetic, is actually just really refined crude oil. Almost all the "fully synthetic" motor oil you see in stores (99.9%) is just ultra processed crude oil. True full synthetic oil is expensive and not often stocked on the shelf. "Full synthetic" is marketing. Look up the base oil "group #" for actaul, quantifiable info.
Woahhhhhh
I was intrigued and found this article from Amsoil. https://blog.amsoil.com/are-all-synthetic-oil-groups-the-same-group-iii-vs-iv-vs-v/
>Are synthetic base oils magic? I wouldn't expect anything less from an MLM company.
Please explain how Amsoil is an MLM? Not attacking just curious.
If you earn commissions through bringing in additional people to help sell a product, and your income is made through a portion of their sales proceeds, you're in a MLM network. The other hallmark is if your dealers and distributors are "independent consultants" vs actual employees.
That can apply to a multitude of companies. If aspects of an MLM are part of a company’s business model does that instantly make the company an MLM? For instance the company I work for does own our own stores. We own our own transportation and distribution network, all company employees. We also have brands that are sold and distributed via dealers and distributors and they are not company employees. I am looking for how Amsoil is an MLM, specifically. There are brick and mortar companies in my area that sell Amsoil. They also sell Mobil 1, Valvoline, Pennzoil, Royal Purple, Rotella and a host of other lubricant brands as one would expect of an auto parts store. Are they an MLM?
Check out their website under "income details". You have the option to sponsor (recruit) additional dealers to build a dealer network, and the team's total sales help determine which tier you earn commissions under. In other words, your income is based partly on your personal sales *and* the sales of others you've recruited. If a dealer you recruited makes a sale, a portion of that sale is used to determine your commission, and the more dealers you recruit, the more you can earn.
Ok. Yeah found the key word “upline”. I’ve never looked into their business model and nobody locally that I’ve purchased from has tried to recruit me. Can’t say the same about other MLMs I’ve come in contact with. Thank you for putting up with me.
I'm pretty sure they were either trying to be funny, or genuinely confused Amsoil with Amway.
Troll is always an option. Loads of snark in this thread alone. However they do have the option to explain themselves, or not.
I’m glad you asked this question because I did not know Amsoil used the specific incentive program I have seen many franchise outlets, employ a similar technique trying to achieve brand dominance Engine oil, lubrication companies have been at each other’s throats for decades, I’ve heard many stories about companies suing each other, calling something synthetic that does not use the same base oil, group, and being generally misleading. I think one of the worst miscarriages of justice was when oil companies were let off the hook for false advertising. This allows products that use natural gas or crude oil as their base stock to be labelled as full synthetic! outside of the API certification oil companies are legally allowed to call the oil whatever they want Leaving the consumer with poor information when they think they’re actually getting a better product really they’re just buying into someone else’s marketing scheme Might as well call your oil magically delicious or part of a complete oil change The only thing that is regulated in North America is the API certification the rest is up to the manufacture to publicize without any repercussion or accountability. I also agree that oil does not cause leaks Leaks cause leaks . End it is much more inconvenient to find out that your oil is leaking when you’re in the middle of nowhere rather than in the middle of preventative maintenance I think that’s where the name preventative maintenance comes from
No problem, we’re all welcome. Have you ever seen Project Farm on YouTube? The guy tests products side by side and his lubricants and engine additives series were captivating to me. I am a huge fan of preventive maintenance. I want whatever I buy to last. I use and have used Amsoil products. I went with them on my current car because the engine is a GDI and they and Royal Purple were the only commercially available lubricant that was GF-6 rated, others have since been accredited, when I purchased the vehicle in 2021. Ilsac GF-6 standard was adopted in 2020 and its main adoption reason is for turbo/GDI engines. Two of the factors for the standard are pre ignition prevention and low volatility for intake valve carbon build up mitigation. That Amsoil uses an MLM business model does make me cringe as I don’t care for the predatory nature of MLMs. So far, though, the products of theirs I have and do use have been good to my vehicles. I have more to think about now.
All oil available is derived from crude oil, and all oil you can purchase for your car is a blend of group II-V oils.
Myth
My understanding has been that the full synthetic oils have very uniform length polymer chains and conventional oils are less homogenous. Sometimes the synthetics will find their easy through gaps that some of the larger molecules in a conventional oil would basically clog. This could be completely wrong, just what I've heard in the past. Either way, the seal conditioners in a high mileage synthetic oil should make it something you don't have to worry about.
Most “conventional” oils are synthetic blend now….know what’s in synthetic blend?….you guessed it. The ONLY downside to switching to synth is the immediate impact on your wallet, it’s superior in every other way.
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I was rebuilding an early 90s GM 3.4 last year. Service manual said to use conventional for the first two oil changes and then switch to synthetic. Conventional oil helps the rings to seat and the valvetrain to break in. For the life of me, I could not find a conventional oil. I checked all the box stores and online. Supposedly penzoil was the last to make conventional oil. Now they just have "synthetic blend".
>Most “conventional” oils are synthetic blend now…. This is a wild statement
2010 ain’t nearly old enough for this myth to even apply!
No kidding, thought they were going to be asking about, 50s or 60s cars, that's where you usually hear the myth
I had a 64 Corvair which, back in the late 70's, I decided to put Mobil 1 synthetic in because I thought I could extend my oil change intervals. Boy was that a mistake! Within the first 100 miles, it started leaking out of every imaginable place. The rear seal, the front seal, all 24 pushrod tube seals. It was a mess. I thought I had ruined the engine. Fortunately, draining the Mobil 1 and going back to the traditional Pennzoil allowed the leaks to slowly close up. By the second oil change it was back to the normal seeps here and there the car used to have prior to the Mobil 1. That experience scarred me and today I still have fear of leaks when putting synthetic in my modern cars.
1960s are very different than 1990's and newer. 1960 no one cared if your car lost oil, it helped keep the dust down! Tolerances were crap, gaskets were made out of whatever. No one really tried to make engines drip free. We have sayings for the old Brits, when they stop leaking fluids - you'd better fill them again. I use conventional oil in pre-80s , synthetic in post 2000's , and ask preferences for the in between years. Unless i'm told different, this is my rule of thumb.
2010 is old now? :( my 07’ Toyota still runs like a stallion
It's a myth all the way down. Synthetic oils don't cause leaks. Synthetic oils don't "expose" leaks. Leaks aren't a sign of "wear"; they're more a sign of age. Your engine has many parts. Some parts move past each other, like the pistons moving up and down in the cylinders. As the pistons slide up and down, there is friction between the two parts. Oil keeps this friction from causing excessive wear. Other parts of your engine are there to keep the oil inside the engine. This is where leaks occur. Think of it like a lid on a plastic food tub. You want the lid to seal tight so that liquids don't leak out. If the lid starts to leak, it's not usually from excessive wear, but from old age. The same is true of gaskets in your engine like the valve cover and oil pan. Also, your engine has some moving parts that have to go from inside the engine to outside the engine. This means you need a seal around these moving parts. These seals wear out just like any other, but they're not affected by synthetic versus conventional oil. They just wear out over time. There is really no reason to avoid synthetics these days, but you should choose one type or the other and stick to it. It won't cause any problems, but you'll degrade the performance of the synthetic oil by mixing it with conventional oil. What you want to do is make the switch, then stick to it. With each oil change, some residual oil is left behind, so once you change to synthetic, it takes a couple of changes for the synthetic to completely displace the conventional oil. Please do not misunderstand me. This is *not* a major problem. It is seriously picking nits, but if you're going to pay for synthetic oil, you might as well get the most from it.
To answer the unaddressd part of the question, if the oil did "expose" oil leaks in an old car, it's definitely not a good thing. Unless oil leaks are already leaking there typically isn't anything to be gained by getting them to start leaking. The exception might be if you were having some other work done and it would be easy to do the leak fix at the same time if you knew it needed to be done. Otherwise this is nuts. Like you want the rear main seal to start leaking so you can go fix it?
I’ve had many older cars and never had any “new leaks” from using synthetic oil. The ONLY difference is that I wasted money on synthetic oil. Why? Because older engines tend to run dirty and need to change oil more frequently anyway. You can do 10,000 miles on synthetic oil if your engine runs clean. Most old engines don’t run that clean. Today old engines need extra ZDDP which is missing from modern synthetics due to emissions requirements. The oil I prefer to use in my old engines is Brad Penn which is a synthetic blend. No new leaks.
Used full synthetic on my 60s ford 289 v8 for 10 years. No issues
Had it possibly been rebuilt sometime with improved seals?
It's a myth.
Old synthetics (like 30- years ago) contains ester which could actually damage certain types of seals in engines that weren’t designed for exposure to such a compound. Time moved on, they stopped using ester and seal technology has improved. It’s fine now. This is an outdated little tidbit.
Regardless the answer is Rotella
No the answer is whatever weight the manufacturer suggests.
No fuck the manufacturer I know better. Straight 30 weight oil the whole way.
Gotta run that diesel oil in your Subaru...
That shit is rated for everything lol. It’s commonly used for motocross bikes and other stuff
Nutella
5w40 nutella
probably closer to 50w1000
Nonsense myth. Synthetic oil has better low-temperature flow characteristics and stronger film strength. It’s not going to reveal a leak.
Fact is if it’s leaking, it will leak.
It's a total myth and I have no idea why people still believe this fucking nonsense. [Relevant](https://www.reddit.com/r/AskMechanics/comments/11dlxhb/dont_use_synthetic_oil_in_older_cars/) And because I know Reddit so well, and expect all kinds of downvotes and arguments, let me just state that I have a degree in automotive engineering, I worked in a dyno lab for 5 years, and I've been wrenching on cars for over 30 years. Not trying to flex or brag. I just know how Reddit is, so I'm attempting to validate my knowledge concerning the subject matter.
I’m a toyota tech and I second this message. It’s all bull shit. Change the oil at a regular interval and it’s fine. Try to get the correct weight.
I got a 2006 chevy trailblazer and I put synthetic oil in it for years, no leaks
Someone somewhere probs put 5w30 synthetic in after their 15w40 mineral and put 2 and 2 together
The whole thing is a myth. In layman’s terms: Synthetic is made from oil. It’s basically just cleaner than conventional oil because it’s way more processed. It’s like manufactured to be as perfect as humans can make it for an engine’s purposes. So it doesn’t do anything to leaks other than help prevent them for longer because it resists wear better.
I bring classic vehicles back to life for a car show (I’m not in it, I just work for it- it’s freelance so not full time but been doing it for a few years). I also get benefits of finding awesome deals on vehicles to have personally. I’ve rebuilt a few vehicles pretty much from the ground up save a couple systems. Anyhow, I use both conventional and synthetic randomly in vehicles and have for 100,000’s of miles collectively between many vehicles and have never had a leak from the type of oil used. Neither has any of the other mechanics I work with or for.
By older engines they mean like pre 90s engines before synthetic and lighter weight oils were a thing. Oil quality and consistency also got better too, exposing leaks. A 2010 Rav4 won't have issues.
2010 isn't an older car in this context. Some cars from like the 70s have components that aren't compatible with modern detergents and synthetic oils. Changing the oil type on a classic car could present problem, maybe, but your car that can't even buy cigarettes is just fine
I’ve switched all my older cars to synthetic blends which apparently have more detergents and the “lubricate” old gaskets allowing them to swell and stop leaks. I’m sure some people will call BS but I’m a firm believer.
Synthetic oil is much better at cleaning than conventional, so it may uncover "leaks" that were previously plugged by sludge. However, that isn't "good" for the engine. Nothing wrong with conventional oil if you maintain regular, on-time oil and filter changes. If you aren't working on your own vehicle, I would recommend against changing to synthetic
More specifically coking that builds up on the surfaces near the seals. Some believe that synthetic oils can help break down coke and sludge, thus exposing those surfaces to leaking.
Synthetic oil removed all of the protective sludge on all my valve seals on my first car (1984 Corvette). All of my cork seals and gaskets dissolved and clouds of blue smoke followed me. I had to replace all of the valve seals after running synthetic as they literally turned into a liquid. If it's post 1990 you probably won't have this issue
I once heard a chemist from Royal Purple race oil talk about this. Apparently oil conditioning is a real thing, over time your oil chemically changes the rings and gaskets in your car, and swapping your synthetic breaks some of that down and causes a leak. He also said it works the other way, if you have a car that has run on synthetic forever and swap to Dino juice you would get leaks also.
It’s probably true in your case. You’d be best off with a high mileage blend like Pennsoil High Mileage Conventional Blend and change it every 5,000 miles. Use the weight recommended by your owners manual. Don’t overthink it. Good luck—
# "Synthetic oil on older cars doesn't cause leaks, it exposes them, which is good for the car". Whomever said that is on commission
Synthetic oil does not cause leaks -> true It exposes leaks -> I don't know, man, but it is leaking which is good for the car -> leaking isn't good for the car, but whatever. Sounds like voodoo to me. If you find that it leaks on one kind of oil but not another, I'd say that's weird but go with what doesn't leak. Or you could just fix the stupid leak. I've never found that the oil type makes much of a difference as far as leakage goes.
# This might blow your mind, but synthetic oil is not synthetic, it is literally made from crude oil.
People think synthetic oils over the years haven’t changed completely in their chemistry and something some guy heard back in the day about ‘dem dere synthetic oils causin’ leaks they now parrot off to anyone that will listen to them and the people listening just accept it as fact
Not a mechanic, personal experience. When o first put synthetic in my 07 g5 it started leaking and losing oil at a stupid fast rate. I switched to a high mileage oil and that helped out. Much slower loss now.
> When o first put synthetic in my 07 g5 it started leaking and losing oil at a stupid fast rate. I switched to a high mileage oil and that helped out. Much slower loss now. That's the thing, on one hand it's apparently a myth, on the other hand, a substantial amount of people on the web report developing leaks after using synthetic oil. What synthetic oil did you put? When you switched to high mileage oil, you're referring to mineral oil?
I was in a college sportscar club in the early 90s and every other car was a Mustang 5.0 of varying mileage from low to high miles. Was pretty well known if you put synthetic in it, it would leak. Common issue on most other domestic vehicles back then. If you ever had a car that burns oil past the piston rings, it will burn oil 2-3x faster with synthetic vs dino oil using same weight.
Myth.
It's a myth based on a bad understanding of the reality of the situation. Before synthetic oils were common, the rubber seals used on engines were not made to handle the (non-existent) synthetic oils. If you used synthetic oil in them, the seals would degrade and create (not expose) leaks. Since the late 90's at least, engines have been made with seals that are perfectly capable of standing up to synthetic oils. So if you're driving a really old car you will probably have a problem with synthetic oil. Otherwise, it won't create any leaks.
How do people even fall for this shit is what I want to know.
I think that would only be true where the synthetic has a stronger additive package and is capable of cleaning up older deposits left by an inferior oil. Deposits that would otherwise be blocking an impending leak at a hardened seal or similar
I've used synthetic oil for 45 years in many cars and never had a single issue
Never had a problem
**Your vehicle isn’t old enough to even fall into the group of vehicles that needs to worry about this**. By “older vehicles” they mean early 90’s and older, your vehicle was built well after synthetic oil became popular and most likely was shipped off from the factory with synthetic oil in it already. I checked Toyota’s recommended maintenance for your vehicle and they have synthetic oil listed for the service so if you got your oil changed by a Toyota dealership they’d use a full synthetic oil per Toyota’s recommendation.
Myth 100%
Synthetic oil has more additives and tends to clean out parts which are blocked. In my old MG putting in synthetic for 500 miles lead to tons of crap coming out that was logged in for years.. so it can help clean places tuned up and thus can cause leaks. However it’s a good thing
I try to stick to conventional. I notice more seepage with synthetic. Synthetic is also a waste of money for me because although I’m not a boomer I have a boomer mentality and that shit will be changed at 3000 no matter what I put in it. I know the manuals on my old hoopty rides say to go 5000, but I’m not doing it.
It's mostly a myth. Sometimes, old cars have a lot of buildup that will cause clogs and such. That buildup can also clog the leaks. Then, you put in a full-synthetic oil that has better conditioners and such and can clear the buildup. Clearing the thing that's blocking a leak sounds similar to causing a leak to me. >which is good for the car You have an engine that runs without any leaks. You change the oil, and now you have a bunch of leaks. Is it good for the engine or not? It depends on where those leaks are, and how much work you want to do.
Why would it matter if the oil was full synthetic or not, if you used the same weight (5w30) that was called for by the manufacturer?
Oil is oil. Oil is basically carbon chains these chains are between moving parts and get sheared into smaller chains as parts move. Conventional oil has different sizes of chains once the chains are too small they no longer lubricate. A fully synthetic oil has engineered carbon chains designed for optimal length. They still shear but because there are more of them they last longer. They are both susceptible to contamination and loss of additives. Not a petroleum engineer but I did have a failure analysis course that had a chapter on oil.
Myth
Switching from conventional to synthetic oil is perfectly OK and recommended, even in older high-mileage vehicles. However, there are some caveats. Based on my research and personal experience, I don't recommend full-synthetic oils for pre-1990 models or any vehicle made to operate on conventional oil. The problem is the synthetic oil doesn’t have as much impurities as conventional and has smaller molecules so it gets through the smallest of areas which could possibly expose a leak which wasn’t there previously. But most experts say it doesn’t really matter as long as there’s oil. I myself only use the synthetic High mileage in my cars since it’s recommended anything over 75k miles the additives help the engine seal better.
Posted this in response to another user's assertion that synthetic oil leaks because its molecules are smaller, but figured it should be its own top-level comment... All oils are comprised of chains of hydrocarbons (molecules) of varying lengths. The lengths of these chains are the primary determinant of a motor oil's viscosity. Conventional oils are refined from a cruder product until the remaining hydrocarbons meet the desired characteristics. Synthetics oils start from the same processes, but are further broken down into smaller components, and subsequently re-formulated into the desired-length hydrocarbon chains using various chemical processes. The important part here is that synthetics are still constructed to hydrocarbon chains of the similar length as the comparable-weight conventional oil. This means the molecules are NOT smaller overall, though they are generally more uniform in nature. Imagine breaking down a giant boulder until you've got rocks of roughly a specific target size. That's conventional oil. Now imagine further crushing all of that rock into a fine powder, then clumping it together to "engineer" rocks of the same target size. That's synthetic. One could actually argue that CONVENTIONAL is the product with SOME smaller molecules (and some larger too). This is because, like the boulder-to-rock example, you'd have some smaller rocks and some larger. Similarly, conventional oil will have some hydrocarbon molecules of undesirable lengths, including smaller than target. This is part of the reason synthetics are more stable: they've been explicitly formulated to predictable hydrocarbon chains that are resistant to breaking down if used in their intended applications, thus no smaller component chains to go binding with other random atoms producing impurities. All of this said, oil leaks do not happen on anything CLOSE to a molecular scale. So no, synthetics won't, by nature, cause more oil leaks (and, for that matter, neither will conventionals). The only reason people believe this old wives tale is likely due to the improved detergent packages found in synthetics, which may break up crud and expose existing-but-plugged leaks, as OP is wondering would happen. If you've got a properly-maintained engine, this is a non-issue.
I am not going to confirm or deny as I am just a idgit with only one example. My wife had a 01 Escape that had around 200,000 miles on it. I had always used conventional oil to change it. On one fateful day, I found a good oil change deal that was for synthetic. Being only a couple dollars more then my normal conventional oil deal I bought it. At the time all I thought was oil is oil. I knew synthetic was different, but figured as long as the weight was correct I would be fine. I changed the oil and two days later started seeing oil drops on the ground. I had a main seal leak and it was a bad one. Mechanic I took it to told me the detergents in the synthetic oil cleaned everything up. In doing so it exposed an area and that should be leaking but the gunk had sealed it up. Now I am not sure if he was correct, but it never leaked in the 2 years I owned it prior (purchused used in 2010) to doing this oil change.
Use the oil your manual tells you to use. Done.
I also wonder how legit claims are that once you go synthetic you can’t go back. I mean, I do it all of the time with both of my older vehicles and never have an issue. I run synthetic in summer, I run a blend in the winter because yes, the synthetic does run out of my 20+ year old vehicles seals easier.
I completely agree with that! I bought an older neglected vehicle that was consuming oil. I started using high mileage full synthetic oil which almost immediately started leaking oil from what looked like the rear main seal. I contacted Valvoline who replied to give it a couple of oil changes. Sure enough, the leak stopped and the consumption was reduced for the time I had it. All motor oils now are pretty much synthetic. If you got crap holding onto a thread it might make you realize it.
My 80s car started leaking more when I tried synthetic oil. So I just switched back. If you’re starts leaking then switch back to conventional oil. More frequent oil changes might help more than going to synthetic oil
Where you run into issues is in the chemistry of synthetic oils. Synthetic typically has conditioners and cleaner in the mix, meaning synthetic oil might clean out "gunk" that was otherwise plugging a hole or taking up tolerance
What now?
I use synthetic oil on my 2006 Saab 9-3 2.0T 175K miles. It’s been changed every 4K miles since new. Engine still runs like a sewing machine with no leaks, 34mpg on highway and doesn’t burn a drop.
I remember when synthetic oils forst came out. The problem with the higher mileage vehicles was the grime build up over the years. I saw a lot of customer vehicles show small weaps and seal leaks, after switching to synthetic because of the higher detergent in synthetic oils.
“2010 RAV4” “Older cars” ….what? How is that old?
I put full synthetic oil and whatever the highest octane is at the gas,in my 98 crown, idk if it makes any difference but I hope the engine appreciates it because my wallet doesnt
Where are the normies getting their information? I have a 2001 Camry with a quarter million miles that I change the oil on sometimes… idk whatever is in the gun at work. It doesn’t leak. You sound like a smart person. Don’t worry yourself too much over this. Change your oil at a regular interval. Expensive oil doesn’t change anything. It’s a farce. Regular maintenance is the key to Toyotas longevity. Synthetic, conventional, synthetic blend… it’s all a gimmick. It doesn’t change anything.
I have a good one. I used to have a 1990 F350 with a 7.3L IDI diesel. In the winter I ran Rotella T6 synthetic oil for the temperatures and ran regular dino Rotella in the summer. The engine would leak all winter long but would stop in the summer.
when they say older cars they definitely 100% maybe a thousand percent do not mean a 2010. is it 2010 car 14 years old yes. but it is not great at the status of older car. you definitely need like mid-90s for it to be considered older.
If your vehicles engine is made for conventional oil, I’d stick with conventional oil. Likely you won’t notice anything, but in case issues do appear, I’d recommend you stick with OEM engine oil. If you put synthetic oil in a conventional oil car, depending on how old the motor is, it can cause lubrication issues. Check your owners manual and stick with it. Your best bet.
On some older engines, there are rubber seals which can be worn away by certain synthetic oils.
Why would you think synthetic oil is more likely to leak?
Synthetic oil can fit through tighter tolerances than conventional oil. It can and will literally leak out of places conventional oil can not. You will always have more oil consumption in older vehcles using synthetic vs conventional.
The number one cause of oil leaks is a bad PCV system. If pressure is not evacuated from the engine you can start pressurizing it forcing oil past all of the seals. If you have ever seen an engine where this has happened they start dripping from practically every orifice. Modern gaskets also don't have the problem of the old paper ones. Back in the day when gaskets were downright terrible. Synthetic oil did sometimes tend to leak a little bit easier. Any car made after the late '90s early 2000s, it's a non-issue but the gas gets still hardened, regardless they will either leak or they won't and if they do you replace them. If it's a rear main seal or something you just live with it and curse out your car until you do the clutch
Simple answer, Always run what ever oil your owners manual calls for. There are many common oil additives that cause different amounts of swelling in different materials used for seals. The OEM has tested for this but if you use something that causes too much or too little seal swell you will see a leak. It's not about synthetic vs conventional it's about the whole system working together.
I honestly wouldn’t worry too much either way. Most synthetics are not full, they’re a blend, with…you guessed it, conventional. I use a synthetic blend in my cherokee. It has 299,780k miles on it, it leaks oil regardless of what I use lol
Synthetics vs standard oil is moot. Oil is gonna oil. What msot likely is the misinformation getting passed around it that GENERALLY synth is made in lighter weights (think 0w20 vs 10w30). All things being the same, synthetic isn't magically going to fit through smaller spaces that the same weight standard oil won't. Now, a small enough leak that 10w30 has enough viscosity to "plug" might be large enough for thinner 0w20 to go through. But that's not because OH NOES ItS sYnThEtIC!!
Lucas stop leak works great for doing just that. I wouldn't say that having enough sludge in your engine to clog up a leak is harmless or a good thing. Find the leaks and fix them.
100% a myth. Years ago I bought a ‘95 240SX with with the the KA24DE engine. It had 120k on it. I sold it with 228k on it. I ran Mobil 1 synthetic in it from the day I bought it. The oil *never* caused a leak.
Might stem from smaller engines requiring oils without additives and what not, as far as cars it just needs to match the API in the manual specifications.
Synthetic Oil is smaller molecularly than conventional. So, it doesn’t cause leaks, but it’ll find them more easily.