I have a 12 bolt rear in my 71 with an eaton posi. I was curious as to a good weight of fluid and what additive (if needed) for my posi or what a good brand is for diff fluid that doesnt need an additive. Thanks guys!!

Paul

Saturday I used 2 quarts of Valvoline GL-5 85W-90 gear oil and one GM limited slip additive in my 12B eaton

I use Amsoil 75W90 Severe Gear synthetic oil in most of the rear ends that I build. It doesn't need the additive. If you want to use a conventional oil then any GL5 oil with GM or Ford additive will work fine.

Thanks guys!! Ill try the Amsoil 75W90. Sounds good. Would there be any reason to get a heavier weight oil or is that fine for everything??? Thanks guys!!

I just tried Castrol LSX 90, no additive, seems good so far.

If this were a drag car and you had very high shock loads then a 75W140 might be better, but the 75W90 shold be fine for most applications. Towing would be another case where the 75W140 would work better.

Its been a while, and i thought i had this figured out (not that there is much to figure out)!!!! My dad and i were talking and if i cant get amsoil would ANY synthetic 75W90 work for me (mostly just street driving not the easiest on the throttle and some drag time)??? I am not opposed to gettting amsoil just there isnt any place real close to get it and i dont really feel like ordering it. So....What im asking is, will any 75W90 synthetic work for me and my posi with out getting an additive??? Thanks for the help guys!!!!

Most will need the additive.

Royal Purple claims not to need the additive, but I haven't tried it yet.

Been using Amsoil 75/90 on the street with no additive per BHG's suggestion to me about a year ago when I changed gears.

I am using the 75-140 Royal Purple in a 12 bolt 4.11 posi. Helped quiet it down also. No need for the posi additive, says it right on the bottle.

I ended up goin with the Lucas Synthetic 75W90 that says it doesnt need the additive. I just figured i will try it and it starts to make a little noise ill put in some additive till the noise goes away! Thanks for the help guys!!!