Synthetic oil made my rear main seal leak
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Same happened here. I accidentally bought synthetic for an oil change and decided to use it. About a week later my head gasket started leaking on both sides and I panicked. Before going through all the trouble of replacing the gasket I decided to go back to conventional. After the first oil change the leak slowed. By the second the leak was gone.
And yet all the oil companies encourage, may, almost preach the use and need for synthetic oil.
I've found if the engine has had synthetic from the get go then it is good to continue with it.
If the engine has been using dino oil for more than a couple of oil changes then best to stick with dino oil.
And for those who buy the synthetic blend.... how much of that "blend" is actually synthetic? I've never gotten a solid answer.
My guess is under 10%. Probably less than 5%.
Just a good marketing scam. If you want to run a blend then buy 3 qt dino, 2 qt syn. And "blend" them in the oil pan. That is the only way to know what you are really getting.
I've found if the engine has had synthetic from the get go then it is good to continue with it.
If the engine has been using dino oil for more than a couple of oil changes then best to stick with dino oil.
And for those who buy the synthetic blend.... how much of that "blend" is actually synthetic? I've never gotten a solid answer.
My guess is under 10%. Probably less than 5%.
Just a good marketing scam. If you want to run a blend then buy 3 qt dino, 2 qt syn. And "blend" them in the oil pan. That is the only way to know what you are really getting.
Interesting, never thought it could make a difference on seals. Been running full synthetic for 18 years. RMS finally started a slow leak last year. I replaced it when I replaced the transmission
Same viscosity? Most synthetics come 5W something, find some 10W something and it won't leak. Done deal!
OP
It was the same viscosity. But it was a slow leak just enough to be annoying on the floor of the garage.
Bofa-Deez
TJ Enthusiast
Or use synthetic 15w40. I had a leak with dino 10w30 oil and synthetic stopped it for me. I think my jeep may be dyslexic though.
Bofa-Deez
TJ Enthusiast
I tried synthetic blend once, got two speeding tickets in the first week. Went back to full synthetic and haven't had that problem since.
This is interesting to me... I have been running full synthetic since around 1990 in everything I own that has a motor... (that is a lot of stuff)... This includes our 99 jeep which we switched as soon as it 10K + miles (synthetics are not great for break-in). I've never contributed a leak to synthetics (although I see it possible) & have never had that experience...
I do know via testing full synthetics last longer than conventional oils (not starting a debate on which is better) just via oil sampling tests they last longer... but never had a leak issue I would say was caused by them.... Again this is very interesting... wondering if it has to do with when you switch over...
I do know via testing full synthetics last longer than conventional oils (not starting a debate on which is better) just via oil sampling tests they last longer... but never had a leak issue I would say was caused by them.... Again this is very interesting... wondering if it has to do with when you switch over...
Bofa-Deez
TJ Enthusiast
Okay, I guess you figured I was trolling a bit and I promised myself that I wasn't going to respond with any intelligence to this thread, but I can't help myself. Behind politics and religion, motor oil has the most myths and fairy tales surrounding it for some odd reason. Without getting into molecule size, total base number, hydro-cracking and viscosity calculations, switching back and forth from synthetic to conventional will not make a seal leak (plenty of info on the web,) or stop it from leaking and switching back and forth is totally safe. I too have sent oil samples in for testing for all of my vehicles and there are few logical reasons for using synthetic oil. One such reason is in extremely cold climates (internet has the explanation) and another is for the extended oil change intervals. I use it because I change my own oil and I double the change interval because it cuts the waste oil I have to dispose of in half, but I do not run the oil filter more than 10k miles before changing it.
The only other logical reason to run it would be in newer tighter tolerance engines that specify synthetic, but again, that doesn't mean they would blow up if conventional oil was used. If you use synthetic oil and change at 5,000 mile intervals, that is a total waste of time and money.
Although my little joke about synthetic 15w40 (yes, I really use 15W40
) stopping my RMS leak was actually true, it had nothing to do with it being "synthetic." It just shows that any of us can make observations and connect them to conclusions, but doing so without sufficient research and data is foolish.
The only other logical reason to run it would be in newer tighter tolerance engines that specify synthetic, but again, that doesn't mean they would blow up if conventional oil was used. If you use synthetic oil and change at 5,000 mile intervals, that is a total waste of time and money.
Although my little joke about synthetic 15w40 (yes, I really use 15W40
) stopping my RMS leak was actually true, it had nothing to do with it being "synthetic." It just shows that any of us can make observations and connect them to conclusions, but doing so without sufficient research and data is foolish.
I don't think it is the synthetic per se, more likely the fact it dosen't sludge as much so the additive package is able to clean out the sludge that was keeping the old seals from leaking. Having more control of molecule size could be a factor. Dino has a bunch of different sized molecules that average 10w. while syn has molecules that are 10w. (Extremely dumbed down) so the oversized dino mol can sort of seal in the seal while the syn can't.
Think "Farmer in the Dell". If he sticks his thumb in the wall he plugs the leak. But if he used his pinky then water would squirt thru, and (back to oil) allow the detergents to clean out the sludge creating a bigger leak.
Now, if the seals are in good shape, neither oil would "cause" a problem.
And in fact that is true, just that syn could magnify a seal problem while dino could mask it. Again, extremely dumbed down theory.
Why some seals hold tight for decades and decades while others seem to last less than a year is an entirely different discussion.
And why it sometimes seems to happen the opposite way is yet another thing. In my youth we were to push the new syn oils (they were a whole lot more profitable then) unless the vehicle looked like it was poorly maintained or more than 2-3 years old. Management didn't want to open themselves to liability for engine work from the oil "causing" a leaky seal.
Think "Farmer in the Dell". If he sticks his thumb in the wall he plugs the leak. But if he used his pinky then water would squirt thru, and (back to oil) allow the detergents to clean out the sludge creating a bigger leak.
Now, if the seals are in good shape, neither oil would "cause" a problem.
And in fact that is true, just that syn could magnify a seal problem while dino could mask it. Again, extremely dumbed down theory.
Why some seals hold tight for decades and decades while others seem to last less than a year is an entirely different discussion.
And why it sometimes seems to happen the opposite way is yet another thing. In my youth we were to push the new syn oils (they were a whole lot more profitable then) unless the vehicle looked like it was poorly maintained or more than 2-3 years old. Management didn't want to open themselves to liability for engine work from the oil "causing" a leaky seal.
+1 - I mainly use Pennnzoil high mileage full synthetic, 10w-30 in summer and 5w-30 in winter. My RMS started to leak last summertime, and the correlation was an oil change using Castrol high mileage full synthetic. They were giving out free Need for Speed (video game) hats so I thought I would try some Castrol. No more Castrol.
An interesting side note to this is the case that synthetic gear oil actually makes differential gears run hotter (per testing done by currie iirc). I'd be curious to know if the same effect is happening in the engine, since many parts are cooled by just the oil (rocker arms for example). If so, I'll continue to use regular dino oil.
Bofa-Deez
TJ Enthusiast
Only during the break in of non-lapped non-OEM gear installs does the synthetic gear oil cause higher temps, so my guess is that the same effect is not happening in engines with OEM parts either. Unlike Currie, OEM gears come from the factory pre-lapprd and ready to use.