1974slcvette

I need to replace both tranny and differential oil, and need some recommendations. I have the M-21 tranny. What type of gear oil should I use, brand and weight? Does the Chevy dealership still sell the posi additive?

Thanks, Mark

73'Vette

GTR1999

The tranny and rear end use 90WT gear oil. Any Name brand will do or you can use the GM stuff. The additive should be the GM stuff though and is available from any GM dealership #1052358 about $7.00 a bottle.

Gary

kdf1986

For the differential I think it is 85W and the rating may be GL-5. And all the bulletins say to use 4oz. of the special anti slip differential lubricant.

Any ideas on how much oil the differential holds?

kdf

GTR1999

3.75 pints

Gary

PRNDL

I bought the GM differential lube along with the GM limited slip (aka posi) additive. It actually has a part# and I figured, why not use the GM stuff.

For the M20 trans I used a name brand gear oil - I can't even remember which. I tried synthetic, but I have since read in articles written by supposed experts that you are not supposed to use synthetic in an M20 trans that was built before the advent of synthetic oils. I can't remember the logical reason, but that is the recommendation FWIW.

BB wowbagger

I use a 85-140 oil in my diff and 80-90 in the tranny..

kdf1986

What is the benefit of the 85-140W oil over the 75-90W oil for the differential?

kdf

pws69

Been running Mobil 1 in both for 2 years. Made a big difference in shifting and reducing gear noise. No leaks at all. I wouldn't go back...

Chevy does have the GM Posi lube - I have also purchased it (GM) at NAPA and CAP parts stores but apparently they don't all carry it.

1974slcvette

Thanks for the help to all who posted!!!!

Mark

74 vert

I put Mobile synthetic in my M20 a couple of years ago.
Penzoil 90W with GM additive in my differential.

No problems with either to date.

Paul @ Thunder

I use Redline 75W90NS in my M21.


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