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.