I plan on changing the rear differential fluid in my 1987 Formula Firebird and need to know what fluid to use.

It is a stock rear end, and I know that it likely was never done by the original owner.

Can anyone please tell me what fluid to use? Capacity?

 

You will need about 2 quarts of 80W90 gear oil. If it has a limited slip differential then you will need the GM limited slip oil additive.

 

Thanks.

I have no idea if it has the limited slip differential though. No build sheet or data from original owner.

 

If it has the 9 bolt Borg Warner rear end then it's almost certain that it does have it. If it's the 10 bolt then it could be either way. Do a burnout and see if both tires spin.

 

For the 9 bolt, do not use the GM non-slip additive.

For 9 bolts w/ posi:

Torco A248514CE 85W-140 (need 2 bottles)
Torco AFM0050JE Type F Limited-Slip Additive (1/2 bottle or full bottle, suggestions vary)

 

You will know when you pull the cover. Makes it hard to buy supplies in advance, but you could always buy the additive & return it if you don't have limited slip.

 

Makes sense. I also saw that you can tell if you raise the rear end, spin one wheel, and see if it spins in the same direction.

Is this right?

 

That doesn't always work. Don't trust that method.