M21 Correct Oil

[miket1]

What Oil did GM install in the M21 Transmission at the factory,  was it a GL5 oil, was it a 75w90 .

Thanks

[L78 steve]

GL5 is the grade 75w90 is the weight. You will find both on the same container. That is fine for your Muncie.

[bcmiller]

I always use the GL4 85w90 from NAPA. StaLube I think is the name.

Do not use GL5.

[1968RSZ28]

Any brand-name 75W90 or 80W90 gear lube will work fine - that's what it was designed, developed, and tested with.

Paul

[bcmiller]

I disagree. You don't want to use GL5 or any synthetic lube in a Muncie. GL5 is not good for the brass synchros.

I know about 8 years ago crash was recommending Pennzoil 80/90 gear plus which was also GL4 back then. Not sure on that now.

[m22mike]

X2, no synthetic in a Muncie. Good read

 http://www.widman.biz/uploads/Transaxle_oil.pdf

[miket1]

I completely drained my M21 to install new oil ,it looks like I need  GL4  NON Synthetic, 
 Thanks for the report on the oil variations,  I knew you guys would know.   

[JKZ27]

What is it about a synthetic GL-4 that is a problem? We use a BG Syncro Shift product that looks appealing but now I'm curious.

[bcmiller]

Synthetic just tends to be thinner and is more prone to leak. It's also more "slick" and you don't want that in a Muncie.  It was designed to use regular gear oil.

[rich69rs]

Synthetic just tends to be thinner and is more prone to leak.

Although not transmission related, with both my '66 Chevelle as well as the '69 Camaro RS, after the engines were rebuilt in both cars I thought I'd do the right thing and use synthetics.  (The pros and cons of this has also been previously discussed on this forum.)  In my case, both cars would leak a few drops around the front engine seal - nothing serious, just anoying.  Went back to 10W30 and the problem went away. 

Our old rides are sometimes finicky when it comes to fluids.  Another example is using Dexron II ATF (originally Type A) in the power steering as per original specs and not the over the counter power steering fluid sold at the local parts house.

[Mike S]

"In my case, both cars would leak a few drops around the front engine seal - nothing serious, just anoying.  Went back to 10W30 and the problem went away.  "

 I experienced exactly the same thing after my motor was rebuilt but with my 396 it was the front and rear seals that seeped slightly.
The dino oil put a stop to that.

Mike

[L78 steve]

GL-5 rating does not indicate a synthetic fluid. It can be either. Synthetic fluids tend to leak due to smaller molecules.

[bcmiller]

Nobody said GL5 meant synthetic. 

But nobody recommends using GL5 in a Muncie either.   Do it if you want, but it isn't good for the synchros. 

[Stingr69]

Find GL-4 90W and be done with it.   GL-5 or synthetic is not an upgrade in this old transmission.

[L78 steve]

No one has stated the reason GL-5 is a problem in Muncie's. I will have to assume that it is too slippery for the braking effect the synchro's provide.
I have 3 Muncie transmissions with the GL-5 and I have not experienced any problems.

[bcmiller]

Steve, this is a quote from Red Line.

GEAR AND SYNCHRONIZER WEAR PROTECTION
Most manufacturers of manual transmissions and transaxles recommend an 80W or 90W GL-4 lubricant.

GL-5 gears oils which are required in hypoid differentials are not used in most synchromesh transmissions because the chemicals used to provide the extreme pressure protection can be corrosive to synchronizers, which are commonly made of brass or bronze. Typically, the use of a GL-5 lubricant in a synchromesh transmission will shorten the synchronizer life by one half. The extreme pressure requirements of spur gears and helical gears found in transmissions are not nearly as great as found in rear-wheel drive differentials.

A GL-4 lubricant provides adequate protection for most manual transmissions, unless a unique design consideration requires the extra protection of a GL-5.

Now, Paul C. has rebuilt a lot of Muncies and has not seen any issues BUT, why take the risk?  See this link for his comments.

http://www.chevelles.com/forums/showpost.php?p=7482682&postcount=66

[L78 steve]

Steve, this is a quote from Red Line.

GEAR AND SYNCHRONIZER WEAR PROTECTION
Most manufacturers of manual transmissions and transaxles recommend an 80W or 90W GL-4 lubricant.

GL-5 gears oils which are required in hypoid differentials are not used in most synchromesh transmissions because the chemicals used to provide the extreme pressure protection can be corrosive to synchronizers, which are commonly made of brass or bronze. Typically, the use of a GL-5 lubricant in a synchromesh transmission will shorten the synchronizer life by one half. The extreme pressure requirements of spur gears and helical gears found in transmissions are not nearly as great as found in rear-wheel drive differentials.

A GL-4 lubricant provides adequate protection for most manual transmissions, unless a unique design consideration requires the extra protection of a GL-5.

Now, Paul C. has rebuilt a lot of Muncies and has not seen any issues BUT, why take the risk?  See this link for his comments.

http://www.chevelles.com/forums/showpost.php?p=7482682&postcount=66

Good info. Thanks for link to the trans expert.

[jeff68]

Sorry I'm a little late to the game on this one, but I've had to deal with the issue of yellow metal corrosion and gear lubes at work. 

The issue really isn't GL-4 vs. GL-5.  Some people claim that the GL-5 gear oils attacks yellow metals, but most gear oil manufacturers now use non-active sulfur phosphorus EP (extreme pressure) additives which don't attack the yellow metals.  Also, even if it is the 'old' type additive, yellow metal attack usually doesn't occur until the temperature gets to about 240F - hopefully above the temp that any of your internal gearbox components experience.

The problem with finding a gear oil for use in transmissions is finding one without Limited Slip (LS) additives. My personal experience is that the LS additives are usually what gives you problems with your synchros and causes grinding upon shifting.

So, if you can find a GL-4 or GL-5 that doesn't state that it has LS additives, then you should still be OK using it in your transmission. I have yet to find any GL-5 locally that does not contain LS additives. I used to be able to find Exxon Superflo 80W80 GL-5 without LS additives, but I believe it is no longer available.

I use CRC StaLube GL-4 85W90 gear oil in my M20.

[miket1]

I bought  Brad Penn  Classic  GL4, 80w90,  made for older trans. with Copper, Brass, Bronze  synchronizers, it was easy to find at Summit.