75w-90 for the diff fluid or go to 75w-140 if you tow a lot.. As far as brand, I think you're good with any brand you prefer.. I know some folks like to run the same brand in their diffs, trans and engine oil... I was going to go with Mobil 1 in mine, but ended up going with Royal Purple as I've always had great luck with Royal Purple fluid in my transmissions I've worked on in the past even on FWD cars...