The differentials are not real fussy about the exact viscosity gear lube they run on, just make sure it is a GL-5 spec'd gear lube. 75W-90, 80W-90, 90W-120, etc. are all fine. If you tow trailers much, go for the higher viscosities like 90W-120, 80W-140, or something close. Personally, for the differentials, I prefer a non-synthetic gear lube as I believe the gears run cooler with a dino oil... and so do many of the custom axle builders like Currie.