Robert Butler

Corvette Elders, will switching to synthetic motor oil cause my 350 motor to leak from the engine seals ? One noted mechanic said that it would. My motor is in excellent condition at present.

Thank you.

Robert

Jud Chapin

No. I switched many years ago and never had leaks. Others will say otherwise.

7T1vette

No. There is nothing in full-synthetic oil that will cause seals/gaskets to leak. If your seals/gaskets are old/weak/worn already, it might be an issue, as the molecular size of full-synthetic oil is significantly smaller than the organic oil molecule. If a leakpath will allow a synthetic molecule to leak but not an organic molecule...it won't be because of the oil.

augiedoggy

Full synthetic has much smaller molicules so if you already have poor seals synthetic can be more likely to leak but if your seals are good then no....
Some argue that it can dry up seals.....
Edit I got sidetracked during writing this post and just saw someone beat me to it.

Tim H

So basically if your engine is older and don't leak with regular oil but leaks with synthetic still don't blame the oil?

Shark Racer

The oil didn't cause the leak. The bad seal caused the leak.

birdsmith

I once switched from dino to synthetic in our Rodeo DD years ago (Valvoline). About 1500 miles later half of it had disappeared. The car only had about 30,000 miles on it, and didn't use oil prior to putting synthetic in. The plugs didn't foul, and the bottom of the vehicle wasn't slathered with oil. It just vanished. I change oil in my cars every 5k miles and have no problems, and that was the last time I tried synthetic oil in a daily driver car. To me, it's false economy putting $6/quart oil in a car that can't take advantage of its benefits (300+ degree breakdown temps).

In a race car,fine. If you want a little extra insurance for your high- dollar collector car, fine. But it just doesn't make sense in any other application...

jb78L-82

As other have stated correctly, synthetic oil does NOT cause oil leaks-weak or bad gaskets do.

Synthetic oil has 3 main advantages over dino/conventional oil:

1.The molecules are all similar in size-uniformity! Conventional oil has molecules of all different sizes. The similar sized molecules protect the surface better simply by their uniform nature-LESS wear.

2. Synthetic oil will flow MUCH better at cold temperatures if it is a true Group IV synthetic-PAO derived. Benefit-MUCH better wear protection at startup and cold operation. Try pouring a conventional oil at 0 degrees-MUD. Pour a synthetic at 0 degrees-like water!

3. Synthetic oil has MUCH better thermal resistance. Benefit-tremendous high stability and resistance to molecular breakdown of the molecules at high temperatures. Benefit-Better wear protection at extremely high temps.

Every piece of combustion engined car or equipment I have uses a synthetic oil (Mobil 1). I have been using a synthetic now since 1986 in my 78 L-82 4 speed. It leaked before the synthetic oil changes and still does today. My DD 2001 Pontiac grand prix which I have driven since new (was my company car that I purchased in 2004 with 49,000 miles) was switched to Mobil 1 in 2004 with 50,000 miles on it. The GP now has 135,000 miles on it and does not leak 1 bit of oil and does NOT use 1 OZ of oil between the 5,000 mile oil change intervals. If I switched to Mobil 1 at the 50,000 miles mark in the GP and the oil mysteriously disappeared, I would have driven to the dealership and handed the car over to them.

Last year, I had to replace the head gasket on my 15 HP Kohler lawn tractor engine. I have used Mobil1 in this engine since it was new in 1997. I was astonished at the condition of the valve train gear! Zero wear signs and perfectly clean. The cylinder still had the hatch marks from when it was bored. The engine has over 2,500 hours on it!

Lastly, a good friend of mine from college who happened to be a chemistry major and later when on for his PHD in Organic Chemistry at the University of Chicago and now works at the world renowned Argon Labs at the University of Chicago as a chief scientist definitely confirms my above statements about the molecular advantages of synthetic oil. When i tell him about the naysayers on the forum his response is "they don't have the foggiest idea of what they are talking about"! His comment to me about synthetics causing oil leaks is "no comment-reference my prior statement"

Hopefully this will help some folks!

PS. Mobil1 is $5/QT at Walmart-CHEAP insurance versus Supertech dino oil at $3.50 per QT.

dar322

yes they can cause leaks. same can happen with the oil additives. at least in my experience they have.

540 RAT

It can rightfully be argued that synthetic oil "itself" doesn't cause leaks. If an an older engine leaks with synthetic oil, it is because of the old seals weren't in the best shape. But, to answer your question about whether or not your engine will or can leak with synthetic, the answer is yes it certainly can.

I've experienced this first hand in a 283 I have. It had been rebuilt maybe 10 years or so ago, and has very little mileage on it, since it is in a weekend toy. It never leaked any oil with dino oil, but as soon as I switched to synthetic oil it leaked. And it doesn't matter if we want to point fingers at the seals or the oil. The fact is, it DID leak with the synthetic even though it never had with the dino oil. I put dino oil back in it and no more leaks. We can debate the pro's and con's of it all, but at the end of the day, those are the facts in my case with the 283.

And if the leaks weren't bad enough, the hydraulic lifters would also bleed down after sitting for months and months with synthetic. Then on start-up it would CLACK, CLACK, CLACK for a good number of seconds before they refilled enough to quiet down. It never did that with dino oil, and never did it again after the dino oil was put back in. So, that pretty much confirms the molecule size discussed, because the viscosity was the same between the synthetic and the dino oil. And the temperatures in SoCal were pretty much always in the 60's and 70's, so no extreme cold conditions to make any difference.

Bottom line: Your 350 may well leak with the synthetic oil, that you did not experience with the dino oil. To know for sure, you'll have to try it and see.

540 RAT
Member SAE (Society of Automotive Engineers)

540 RAT

This is what you often find when researching oil on the Internet and elsewhere. Problem is, the oil itself doesn't read that stuff, so it doesn't know it is supposed to do that. I wanted to see for myself how true those claims might be, since there is a ton of misinformation out there about motor oil.

My own real world oil testing does NOT support that over stated claim. Here's what I'm talking about. I heated a number of oils to the temperature at which they started to vaporize/smoke, which indicates the onset of thermal breakdown. Thermal breakdown is the point at which the composition of the oil begins to change due to the temperature it’s exposed to. After oil reaches thermal breakdown, the lighter oil fractions will vaporize, leaving thicker and heavier oil, contributing to poor circulation, reduced fuel economy, increased oil consumption, increased wear and increased emissions. You obviously want to avoid reaching the point of thermal breakdown.

Here are my test results:

5W30 Pennzoil Ultra, API SM = 280*

5W30 Mobil 1, API SN = 265*

10W30 Lucas Racing Only = 290*

5W50 Motorcraft, API SN = 275*

10W30 Amsoil Z-Rod Oil = 300*, the BEST in this test

20W50 Castrol GTX, API SN = 275*

5W20 Castrol Edge w/Titanium, API SN = 280*

10W30 Joe Gibbs XP3 NASCAR Racing Oil = 280*

5W30 Castrol GTX, API SN = 280*

10W30 Valvoline VR1 Racing Oil, silver bottle = 260*, the WORST in this test

0W30 Brad Penn, Penn Grade 1 = 280*

5W30 Royal Purple XPR = 285*


Here are the “averages” for the onset of thermal breakdown with these 12 oils:

Full synthetic oils = 282*

Semi-synthetic oil = 280*

Conventional dino oils = 272*

For the oils tested above, certain specific oils did show a significant difference, such as the synthetic Amsoil Z-Rod oil which had a 40* advantage over the conventional Valvoline VR1 Racing Oil.

But, as for overall averages, there was only a 10* difference between synthetic and conventional oils. So, the real world observation here does NOT support common internet oil info claims about synthetic oils having an unbelievably high temperature capability compared to conventional oil.

Don’t believe everything you read on the internet about motor oil. Because there is a lot of misinformation floating around, that has often been repeated over and over. Most sources never ever do any independent testing at all, they just repeat what others have already written. And it doesn’t matter how many times, different sources repeat the same wrong information, it will never magically become true.

540 RAT
Member SAE (Society of Automotive Engineers)

hugie82

I use synthetic in most of my vehicles but on 2 occasions with high mileage (over 60k) I had to replace seals to stop the seeping. You won't have an instant leak but old seals might start seeping.
No, not all old motors do it but you can always switch back to Dino oil if it does.

jb78L-82

Which synthetics above are Group IV true synthetic? Most US marketed "synthetics" are NOT true synthetics- they are highly refined dino oils group III. Been that way the last few years since the court case brought by US oil companies. Am soil/Royal Purple are a Group IV! Mobil 1 5W-30 is a group III dino oil. Euro Formula Mobil 1 0W-40 and 5W-50 in the US are the only Group IV synthetics in the US only.

dtamustang

synthetic oil (at least to me ) is throwing money down a hole , unless you are racing.
i have never used the stuff and have never had a problem by not using it.
whether it causes leaks or not......i have heard the science types say it will not and their arguments make a lot of sense........i have seen oil leaks appear that were not there before, and i have seen oil leaks disapear when switched back to conventional oil. ( a friends truck). the only thing i know for sure is that it leaked with synthetic and did not leak with conventional.

Tim H

So in order to run synthetic oil you may have to rebuild your engine?
Doesn't sound like a very good trade off when the old style oil was working fine?

calwldlife

freeway speeds and my oil temp gauge reads 250 plus.
even 10 deg for US synth is a welcome cushin.

Shark Racer

What kind of margins are we talking about here? An oil that wouldn't have leaked will start leaking soon as the seal wears just a bit more. In this case, we're talking about prolonging the inevitable.

Also, re-sealing a motor is not necessarily the same as rebuilding it.

augiedoggy

Exactly...... its kinda like the thick "motor honey" additives designed to "plug" the porous bad seals that cause leaks.... oil in smallest form is bigger than synthetics which lubricate better in part because it can fit in smaller crevices say in a cylinder wall and stick or stay in place better to lubricate... it also stays between tighter fitting rubbing parts better because of this.
A crude comparison..... would you rather have powder suger or granular sugar stuck between your toes or ..... one would wear at the skin quick and the other may actually lubricate... however the powder sugar would leak through a sock before the granular stuff would.

Tim H

So replacing that rear main seal without new main and rod bearings would be OK with you, and doing it under the car.
Thing is if my car didn't leak now I'll be damned if I would change oils just to see if it would leak!

augiedoggy

If it leaked you could always just go back to Dino oil... its not like it damages the seals...it just reveals the weakness in them if its there.