im swapping a damaged rear axle out of my 91 rs with a new set off ebay. im wondering what type of replacement differental fluid i should use and how much? thanks

One of the beter synthetics, such as Mobil1, Amsoil. Red Line, etc. 75W-90 or some such. The exact numbers aren't real critical. NOT the super thick stuff like 85W-140.

i use M1 and amsoil 85w-90 with no problems. if you have a clutch type posi use the gm posi additive.

85W140 is just VERY slightly (only ONE step!!) thicker than 75W90, and is a very good choice for our axle's!!
85W140 synthetic has almost the same weight as 75W90 dino!!(but is superior in every way!)
Synthetic oils are due to the way they are made always slightly thinner when compared to a not synthetic oil of the same specifications.
You just can't compare weight specs. of engine oil with gear oil!
F.i. XW90 gear oil compares to Xw40 engine oil and XW140 compares to XW50 engine oil (you see, only ONE step.....)
Know what you're talking about before giving any advise!!!!

I have a question related to this, I have read somewhere that with the top of the line synthetic rear end lubes, ie. Royal Purple, Red Line, etc., the posi additive is not required. Is this a true statement or is it bogus. Thanks in advance.

that's what they say but for 6 dollars a bottle i'll use the gm posi additive reguardless of what the oil makers say

when i put in my slp zexel torsen, do i need to use the posi additive, cause it doesnt use clutches does it?

don't need the posi additive if you don't have clutches.

Please be advised that GM did a service bulletin on this issue. I believe that it is floating around here somewhere. But alot of people were complaining of noise coming from their rears with the torsen differential. So GM started advising customers to use the GM limited slip additive with the torsen differential.

Oh yeah, it even says that on the torsen website. Zexel torsen advises the use of the GM limited slip additive with the torsen because of this issue.