Diff Fluid and Additive
Thread Starter
Instructor
Joined: Jun 2019
Posts: 232
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Diff Fluid and Additive
I know it's probably been asked a hundred times.
Changing differential fluid on my '81. What is the best fluid and additive. Shop manual says 4 oz additive and top to fill plug. Capacity says 4 pints.
Changing differential fluid on my '81. What is the best fluid and additive. Shop manual says 4 oz additive and top to fill plug. Capacity says 4 pints.
Race Director
Joined: Apr 2003
Posts: 19,351
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From: Tampa, Florida
Any high quality gear oil will work and use GM additive. High quality means any of the big names, Castrol/Mobil etc. Skip over the house brands. Something I do is drive until the diffs hot, drain (actually suction out) and refill then drive again until its hot the drain and refill a 2nd time. Surprisingly the fluid will be very dirty on the 2nd drain. Considering how how often diffs need service the added cost is minimal.
Le Mans Master
Joined: Sep 2001
Posts: 7,739
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From: Unreconstructed, South Carolina
'81 vette specs direct from GM ... free pdf download ... axle specs p55 of 75-page pdf
https://www.gmheritagecenter.com/doc...t-Corvette.pdf
https://www.gmheritagecenter.com/doc...t-Corvette.pdf
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,810
From: Cool Northern Michigan
I agree. If the original lube worked really well for 40+ yrs, it will work another 40.
The GM axle lube and Posi Modifier can be had from Muskegon Brake and even Amazon has it.
2 Lg bottles lube
1 small bottle of additive.
The GM axle lube and Posi Modifier can be had from Muskegon Brake and even Amazon has it.
2 Lg bottles lube
1 small bottle of additive.
Team Owner
Joined: Jun 2001
Posts: 23,375
Likes: 1,066
From: Virginia USA
Once you get it full with oil and additive (GM of course) take it to a parking lot and do a few "fun" figure 8's to slosh the additive and oil around to mix it real good. Do this after driving a little to get the oil temp in the diff up.
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,810
From: Cool Northern Michigan
eddp,
If you go to a food service store like GFS, they have a clear plastic squeeze bottle such as those used for BBQ sauce etc. You can do all your mixing of the two fluids right on your workbench. May take three times and I believe the total fill was around 56 oz. The condiment bottle fits up into the differential area easily for dispensing.
Smear a little thread sealant on the plug and you are good to go. That fluid will be mixed thoroughly by the time you back out of your driveway.
If you go to a food service store like GFS, they have a clear plastic squeeze bottle such as those used for BBQ sauce etc. You can do all your mixing of the two fluids right on your workbench. May take three times and I believe the total fill was around 56 oz. The condiment bottle fits up into the differential area easily for dispensing.
Smear a little thread sealant on the plug and you are good to go. That fluid will be mixed thoroughly by the time you back out of your driveway.
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Last time i used 2 bottles of this, think i bought a box of 6 way back when.
Last edited by Mod75; Oct 15, 2019 at 08:25 PM.
Le Mans Master
Joined: Dec 2006
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Any high quality gear oil will work and use GM additive. High quality means any of the big names, Castrol/Mobil etc. Skip over the house brands. Something I do is drive until the diffs hot, drain (actually suction out) and refill then drive again until its hot the drain and refill a 2nd time. Surprisingly the fluid will be very dirty on the 2nd drain. Considering how how often diffs need service the added cost is minimal.
