The tranny holds just a little less than 2 quarts. 
M21 Lube Change
Thread Starter
Instructor
Joined: Feb 2008
Posts: 237
Likes: 0
From: Roseville CA
M21 Lube Change
While my car is up on blocks, I'm changing out all fluids.
How many Fl. oz. of gear lube does a M21 take? I don't want to over buy.
From what I have read, you use the same GM Axle Lubricant SAE 80w-90 as you do in the rear diff. Correct?
Do you need to add the 4 fl. oz. additive as with the rear diff?
Thanks
How many Fl. oz. of gear lube does a M21 take? I don't want to over buy.
From what I have read, you use the same GM Axle Lubricant SAE 80w-90 as you do in the rear diff. Correct?
Do you need to add the 4 fl. oz. additive as with the rear diff?
Thanks
Drifting
Joined: Oct 2007
Posts: 1,254
Likes: 5
From: Brown Summit North Carolina
You do not use the additive as in the rear end.
As for the right amount. Buy 2 32oz. bottles. It won't take all but will take more than 32oz.
I used Royal Purple and have been pleased.
The item # is 01301. It is called Max Gear 75w 140 can get at Auto Zone.
A tip on getting it into the trans.
Buy about 7' of clear 3/8 id vinyl tubing. Feed the tubing from under the open hood and stick a funnel in it.
Then have a helper pour the fluid while standing at the P/S fender while you are under car holding tube in fill hole. Just fill a little beyond full and remove tube and allow excess to drain into a pan.
Once all excess has drained to bottom of fill hole your done.
David
As for the right amount. Buy 2 32oz. bottles. It won't take all but will take more than 32oz.
I used Royal Purple and have been pleased.
The item # is 01301. It is called Max Gear 75w 140 can get at Auto Zone.
A tip on getting it into the trans.
Buy about 7' of clear 3/8 id vinyl tubing. Feed the tubing from under the open hood and stick a funnel in it.
Then have a helper pour the fluid while standing at the P/S fender while you are under car holding tube in fill hole. Just fill a little beyond full and remove tube and allow excess to drain into a pan.
Once all excess has drained to bottom of fill hole your done.
David
CF Community Team
Joined: Oct 2004
Posts: 19,366
Likes: 5,244
From: Cape Cod, Mass.
2025 C6 of the Year Finalist - Unmodified
2025 C8 Z06/7/E-Ray of the Year Finalist - Unmodified
2023 C3 of the Year Finalist - Unmodified
2021 C8 of the Year Finalist Unmodified
2020 Corvette of the Year Finalist (performance mods)
2019 C1 of Year Winner (performance mods)
2017 Corvette of the Year Finalist
2016 C2 of Year
2015 C3 of Year Finalist
You absolutely DO need the 4 OZ. bottle of GM posi additive in the rear end but NOT in the tranny. 80/90 gear oil in both the tranny and the rear is what I use.
Drifting
Joined: Oct 2007
Posts: 1,254
Likes: 5
From: Brown Summit North Carolina
Just curious why you had to say so strongly that he should?
David
Pro
Joined: Sep 2002
Posts: 624
Likes: 1
From: Melbourne, Fla. 6 months- New Middletown, Ohio 6 months
Originally Posted by capevettes
You absolutely DO need the 4 OZ. bottle of GM posi additive in the rear end but NOT in the tranny. 80/90 gear oil in both the tranny and the rear is what I use.
In the original post, Ward72 asked if the posi additive was needed in the transmission. Capevettes answered that question. He emphasized that it was needed in the rear end but not in the trannie. I do not see where he was out of line but that is just my opinion.
You absolutely DO need the 4 OZ. bottle of GM posi additive in the rear end but NOT in the tranny. 80/90 gear oil in both the tranny and the rear is what I use.
In the original post, Ward72 asked if the posi additive was needed in the transmission. Capevettes answered that question. He emphasized that it was needed in the rear end but not in the trannie. I do not see where he was out of line but that is just my opinion.
Drifting
Joined: Oct 2007
Posts: 1,254
Likes: 5
From: Brown Summit North Carolina
Originally Posted by capevettes
You absolutely DO need the 4 OZ. bottle of GM posi additive in the rear end but NOT in the tranny. 80/90 gear oil in both the tranny and the rear is what I use.
In the original post, Ward72 asked if the posi additive was needed in the transmission. Capevettes answered that question. He emphasized that it was needed in the rear end but not in the trannie. I do not see where he was out of line but that is just my opinion.
You absolutely DO need the 4 OZ. bottle of GM posi additive in the rear end but NOT in the tranny. 80/90 gear oil in both the tranny and the rear is what I use.
In the original post, Ward72 asked if the posi additive was needed in the transmission. Capevettes answered that question. He emphasized that it was needed in the rear end but not in the trannie. I do not see where he was out of line but that is just my opinion.
Do you need to add the 4 fl. oz. additive as with the rear diff?
This last sentence from his posting indicates he knows that it goes in the rear end.
David
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Top 10 Most Expensive Corvettes Ever Sold on Bring A Trailer
Brett Foote
10 Things Every Corvette Owner Needs (2026 Edition)
Michael S. Palmer
8 Most "Only Corvette Owners Understand" Quirks and Problems
Pouria Savadkouei
10 Reasons the C6 Z06 is Still A Performance Benchmark After 20 Years
Joe Kucinski
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
CF Community Team
Joined: Oct 2004
Posts: 19,366
Likes: 5,244
From: Cape Cod, Mass.
2025 C6 of the Year Finalist - Unmodified
2025 C8 Z06/7/E-Ray of the Year Finalist - Unmodified
2023 C3 of the Year Finalist - Unmodified
2021 C8 of the Year Finalist Unmodified
2020 Corvette of the Year Finalist (performance mods)
2019 C1 of Year Winner (performance mods)
2017 Corvette of the Year Finalist
2016 C2 of Year
2015 C3 of Year Finalist
No offense taken or intended 
Drifting
Joined: Oct 2007
Posts: 1,254
Likes: 5
From: Brown Summit North Carolina
Pro
Joined: Nov 2007
Posts: 741
Likes: 1
From: Woodstock GA
Another tip for getting the gear oil in the trans is with a fluid pump...Valvoline makes one (haven't seen them in my local parts stores lately). You can also get them at Wal-Mart. They're cheap, and easy to use.
