correct tranny oil for C3 4 speed

[Ray G.]

I know this topic has been beaten to death but I have used Red Line MTL in my later year Corvettes but I am unsure what to use on my 69 M21 transmission. Thoughts?

[Michael G.]

Re: correct tranny oil for C3 4 speed

Ray, I don't believe there was/is any "correct" fluid other than a "quality", name-brand 85-90w gear oil for these GM manual transmissions. I would be careful with synthetics which can find their way past the earlier seals.

[Duke W.]

Re: correct tranny oil for C3 4 speed

Going back to the fifties, the GM recommended lubricant for both manual transmissions and hypoid axles was Mil-L-2105 (see your shop manual and owner's manual). This mil-spec was cancelled over a decade ago and replaced with the commerical SAE 80W-90 GL-5 gear oil.

GL-5 has an EP additive that is necessary for the highly loaded hypoid gear set in the axle, and this addtive is what gives GL-5 its pungent odor. There is all kinds of internet blabber about "yellow metal corrosion" caused by the EP additive, however, this corrosion mechanism is only signficant at temperatures well above normal operating temperatures.

I have yet to see or hear of any old transmission that showed evidence of synchonizer corrosion. Has anyone else? Most of our old manual transmissions survived decades of use and abuse on the original factory fill oil until wear, not corrosion, finally forced a rebuild.

It amazes me that these myths persist without any evidence to support them.

Redline MTL is a 70W-75 GL-4 gear oil - same viscosity range as ATF - and is not suitable for a vintage manual transmissions, but is okay for modern manual transmissions that specifiy ATF as the lubricant, but some specifiy 75W-90 GL-4, and an example is Redline MT90.

Some guys use 75W-90 GL-4 ("synthetic") or 80W-90 GL-4 (conventional) in vintage manual transmissions, but I have yet to hear the results of any long term durability testing. GL-4 lacks the EP additive that is necessary for hypoid gear sets.

As with many things in life, if you're not sure, READ THE INSTRUCTIONS! (see paragraph one) Or consider the old adage: If it works, dont fix it!

BTW, brand is not important as long as the bottle has the API "donut" and is clearly labeled 80W-90 GL-5. There is also a "synthetic" GL-5, SAE viscosity range 75W-90, but I don't recommend this unless the car is frequently driven in very cold weather.

Duke

[Ken A.]

Re: correct tranny oil for C3 4 speed

If you want it to shift "like butter", then use GM positrac lube. Also works great in cold weather.

[Ray G.]

Re: correct tranny oil for C3 4 speed

Thanks to Mike, Duke and Ken. I appreciate all your input! Will do!

[Gary R.]

Re: correct tranny oil for C3 4 speed

If you want it to shift "like butter", then use GM positrac lube. Also works great in cold weather.

Hi Ken, .... well that's a new one on me. All the years rebuilding diff's and trans and I never thought to try the additive in a Muncie or ST-10. I now use 2 bottles in the diff's because I believe the additive changed a couple of years ago. I never had a posi chatter in any of the tuned posi's I built until the first time I used 1 bottle of the new additive. I thought the posi tune was off, even though I run them in 700-1000 rev's. I had the fellow add a 2nd bottle and solved the issue.

[Russ S.]

Re: correct tranny oil for C3 4 speed

If you want it to shift "like butter", then use GM positrac lube. Also works great in cold weather.

Ken are you referring to the GM additive or GM lube(oil)?

[Ken A.]

Re: correct tranny oil for C3 4 speed

You need both, also good to use was GM synchromesh lube.