what differential fluid should go in 8-3/4

I run synthetic Amsoil Severe Gear 75w-90. Although brand name conventional fluids offer good protection, synthetics tend to be better and provide extra insurance under heavy duty/performance use. They also tend to run cooler. I have the stuff in my race car 8 3/4" as well as my Dodge dually. The thinner stuff will free up a few extra HP over the thicker viscosities and it has a friction modifier in it for sure-grip/posi rear ends. But one can add their own as well if it needs more. My differential guy likes the stuff as well and I've reused bearings a couple of times between gear changes.

http://www.thelubepage.com/severe-gear-75w-90