Maybe someone can explain this...

[ULTM8Z]

I searched around online but didn't find an explanation... and this is truly a wierd one.

First the background: My 383 seems to consume about 1/2 qt of oil every 7 or 8 months or so. Not sure if it's the PCV system or what, but there doesn't seem to be any negative effects... plugs are clean, no smoke, there hasn't been any leaks, etc... So whatever, I just monitor it and top off as required.

I typically use Mobil 1, 5W30. I didn't have any Mobil 1 to put in the half qt, so I grabbed some Costco Kirkland full synthetic that I had laying around from when I had my previous daily driver that used 5W-30. The ratings and specs on these oils seem to be the same, both GM approved, etc...

I took the car out for a drive as normal, then came back.

Now for the riddle:

When I went back to the car later, I noticed a small puddle of engine oil under the car. I'm like, WTH?

So I jack the car up, pull off the torque converter dust shield and notice its coming out the rear seal... so now I'm like #@%!!!! Seriously on a brand new 383??!!!

But then I notice that the leak is essentially new clean oil, not the dark brown color of used oil that has ~1500 miles on it. Which seemed awful peculiar. Even more peculiar was the coincidence of just having put in an oil I've never used before on the Camaro.

So I drained all the oil, removed the filter. Then went to Autozone and got some fresh Mobil 1 and a new filter and then put it in.

It was still leaking at first, but much less. And with each successive drive, less and less. Over the course of a week now, the leak is completely gone. Scared the crap out of me... I thought I had ruined the rear seal or something.

Further, the oil I drained out was a homogeneous mixture... not like the Kirkland stuff was separated out or anything like that.

So what gives? How can a particular oil essentially initiate its own leak?

 

[72SS9681]

The fact that the oil is "new clean oil" makes me think the oil leaked out before it got to the pan/main seal. Wondering if there's a leak in the rear of the valve cover. When you add new oil the oil backs up in the cover and leaks out the rear of the cover, as the oil leaks down into the pan it stops leaking. Can't say that I've ever seen this but seems plausible.

 

[Zstar]

I use Pennzoil Synthetic in all my engines because of the detergent package. Is it possible the Kirkland has a detergent that Mobil1 does not? I have never been a fan of Mobil1.

 

[COPO]

You’re mixing 2 different chemistries of oil. Who knows what reaction took place but it did.

 

[krabben1]

Your engine has you now.......the game is on.

 

[FS87LT]

I've never been a fan of Mobil1 either, but I doubt mixing the two oils cause the leak. What viscosity oil do you normally use in the engine?

Some brands of motor oils have more, what I term, "lighter parts" in them than others, even at the same viscosity ratings. These lighter parts are more prone to wick through rubberized cork gaskets to cause seeps/leaks. Could do the same with rear main seals, too, I suspect. In the later 1980s, a friend cussed Castrol GTX as it caused his valve cover gaskets to leak, but the Pennzoil he had been using did not.

When I ran that situation by my late machine shop associate, that's when he noted the wicking effect some oils had with cork gaskets. He also noted that the "leaking oil" would get to places that the non-leaking oil would not, for better lubrication. After his explanation, we both laughed.

Considering that Tommy was not brand-loyal to motor oil, just buying what was on sale at the local auto parts stores, usually. Like many old drag racers I've known, he generally used Valvoline, though.

I doubt any damage was done. Just make sure you have some of the desired oil in your garage for future use!

Just some thoughts,
FS87LT

 

[67johnny]

Is it an original 2 piece rear main seal or do you have an updated Gen 2 block with the one piece seal?
Also... what type of Oil Pan gasket?
There are 1 piece gaskets out there that seem to leak less often than the stock 4 piece OEM item.
Looks like this> https://www.summitracing.com/parts/FEL-OS34509T

 

[ULTM8Z]

Is it an original 2 piece rear main seal or do you have an updated Gen 2 block with the one piece seal?
Also... what type of Oil Pan gasket?
There are 1 piece gaskets out there that seem to leak less often than the stock 4 piece OEM item.
Looks like this> https://www.summitracing.com/parts/FEL-OS34509T

It's a HT383 short block, based on the mid-90's Vortec block, so one piece rear seal.

 

[ULTM8Z]

I've never been a fan of Mobil1 either, but I doubt mixing the two oils cause the leak. What viscosity oil do you normally use in the engine?

Some brands of motor oils have more, what I term, "lighter parts" in them than others, even at the same viscosity ratings. These lighter parts are more prone to wick through rubberized cork gaskets to cause seeps/leaks. Could do the same with rear main seals, too, I suspect. In the later 1980s, a friend cussed Castrol GTX as it caused his valve cover gaskets to leak, but the Pennzoil he had been using did not.

When I ran that situation by my late machine shop associate, that's when he noted the wicking effect some oils had with cork gaskets. He also noted that the "leaking oil" would get to places that the non-leaking oil would not, for better lubrication. After his explanation, we both laughed.

Considering that Tommy was not brand-loyal to motor oil, just buying what was on sale at the local auto parts stores, usually. Like many old drag racers I've known, he generally used Valvoline, though.

I doubt any damage was done. Just make sure you have some of the desired oil in your garage for future use!

Just some thoughts,
FS87LT

I've been using 5W-30. I had started with 10W-30 conventional oil for the break-in procedure, but then switched to 5W-30 synthetic after the break in was complete.

I read the label on the Kirkland container carefully before I put it in as I was actually (ironically) a little hesitant at first. But I managed to convince myself that it was probably ok... I mean they sell millions of quarts of this stuff and it's not like people's engines are blowing up left and right.

But your theory about components of the oil managing to permeate seems plausible to me. That thought had also crossed my mind as I had encountered a similar situation back in 2007 time frame where I was noticing a strong "fuel-like" odor in the garage all the time. After ruling out leaks or issues with the CCP system, I finally traced the problem to components of the fuel actually permeating vapors through the braided rubber fuel line I had installed for the EFI system. Ultimately switching to Teflon lines solved the issue.

In which case the gradual reduction to zero of the leak must be due to residual amounts of the Kirkland oil being consumed and/or the remnants of the suspect "component" leaking out.

I agree, I don't believe the seal is damaged either.... but yeah, that was a near heart-attack inducing event.

 

[ULTM8Z]

Hmmm.... I'm still getting some drops of oil, even after the car has been sitting for a week. It had stopped for a bit, but it had come back again.

I read somewhere that a rear seal would only leak oil while the engine is operating. So maybe it's the oil pan gasket then? When I went under the car earlier on it wasn't obvious where it was coming from.

But seems like it can't be the rear seal since the standing oil level is below the rear seal. In which case this coincidence with the Costco oil would just be a red herring...

But if what I read is true, it's easy enough to pull the pan down and replace the gasket... probably give that a go soon. I suppose I could drain a quart and see if the leak goes away.. that might prove it's oil level related and then point to the rear part of the pan gasket.

Maybe right before I plan to park the car for a week again...

The back of the engine block is dry at the valve covers and intake manifold, and I don't see any oil coming out of the rear pressure port. But even those locations can't be leaking after a week of non-operation.

Thoughts?