Will synthetic make an old engine leak?

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Sep 17, 2003
I've heard many times that switching to synthetic in a high mileage vehicle will clean the deposits from the gaskets and could cause leaks.
I don't get that impression from reading this forum.
I'm afraid of switching to Mobil 1 in my zx2 with 86,000. I have always had 3000 to 4500 dino oil changes.

I had a mustang that I switched to Havoline synthetic at 60,000 back in '94. The rear main started leaking.

Am I being paranoid?
I switched over my Grand Prix at 104,000. It's at 109,000 right now, so far no problems.

If you've kept it pretty well maintained, then I think it's not much to worry about. My car was dealer serviced it's whole life, and clean as a whistle, so I didn't worry too much before switching.
Yes and no. Meaning it doesn't "cause" the leak directly. If your gaskets are old or dried out, a "more slippery" synthetic may sneak past them and result in a leak. Likewise, if you have crud sealing the leak, the extra cleaning properties of a synthetic may clean out the stuff responsible for the seal..and result in a leak. I've had a leak in a car with only 13k mi. on the odo after switching to synthetic (old PAO Syntec).

In either case, synthetics don't cause leaks directly and if you do get a leak, simply fix the worn gasket.

simply fix the worn gasket.

Since rear main seals are so easy to fix anyway

I agree, though, just switch. If you start to leak oil, switch back. The absolute worst I'd be afraid of is switching on a car that was NOT maintained and the rings are worn. You'd just eat oil by switching. However, if that were the case, you probably wouldn't bother switching anyway.
Here's the official word from Mobil 1's website. Not sure how much of this is the word of God, and how much is Communist propaganda.

"Is Mobil 1 with SuperSyn™ compatible with the seals and gaskets in new and older vehicles?" "Mobil 1 with SuperSyn™ does not cause leaks. It is fully compatible with the elastomeric materials from which automotive seals and gaskets are made. ExxonMobil engineers are wary of conventional oils that tout their use of additional seal-swelling agents. With extended use, these agents can over-soften engine seals, resulting in leaks. More to the point, an oil additive will not rejuvenate worn or damaged seals. The damaged seal may have been caused by a worn rotating metal component in the engine. If an older engine is in good condition and does not have oil leaks, Mobil 1 with SuperSyn™ provides the same advantages as when used in a new engine. ExxonMobil recommends taking measures to repair the leaks, then using Mobil 1 with SuperSyn™. ExxonMobil also always recommends following the automobile manufacturer's manual for the proper oil to use."

Originally posted by guitargeek:


simply fix the worn gasket.

Since rear main seals are so easy to fix anyway


My rear seal is not that easy to fix.
What about the cost involved...

In answer to the original question,YES it CAN cause the seals and gaskets to leak. And USUALLY, switching back to a dino or a heavier dino will USUALLY fix the problem.
THe reason they do is because (even on a new seal) if there is the SMALLEST crack at all, an imperfection or whatever, the Syn will seep or spill past the seal because it is smaller and because once it makes a path out of the engine, the rest tends to follow.
I've got a Del Sol with 141,400 miles on it. Switched to M1 5w-30 at 131,500 miles. It doesn't consume a drop of oil.

I think a lot of people who say switching to synthetic will make your gaskets leak are still listening to stories passed down from way back when Mobil 1 was 100% PAO (except the Group 1 package). Original Mobil 1 actually would shrink the seals in an older car and cause massive oil consumption. None of the Tri-Synthetic or SuperSyn has that problem.

Originally posted by HOndaGuy:
do an interval with auto rx and dino oil if your unsure of how clean your engine is, then go synthetic.

OK I know this AUTO RX is good stuff. I would be afraid that using it would have the same effect as synthetic oil. If it cleans very well it might start leaking.
HOndaGuy nailed it. IF you are concerned, try Auto Rx first. That stuff will condition seals as well as clean. Hocus pocus to me, but it seems to work.

My van started leaking immediately after using a syn blend for the 1st time at 84K. Switching back to dino did not stop the leak. Used a high mileage oil next. Still leaked. I reluctantly tried Auto Rx and it did the trick. That was @ 7K miles ago. If Auto Rx did not work, I was prepared to spend the $$'s and get the gaskets or rear seal repaired.

As you can see from the various responses so far, it's hard to predict what will happen in your engine.
Each one is unique.
Do a Mix. I sometimes but not always do a 50-50 dino/synthetic mix on my 70k mile truck just so I dont get any suprise leaks. Or as someone stated if it leaks while trying the synyhetic you could return to using the dino & the leak should subside.
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