[MJN]

I would use Mobil 1 if you go with synthetic, otherwise Mobil or Valvoline 80w-90 will work fine. If it is a factory posi and has the little clutches in the diff, then you will need GM's friction modifier. It comes in a 4oz bottle andyou just dump it in with the oil. I found this out the hard way, I tried using a friction modifier from Napa but kept getting a clunk when the diff would engage, after putting in the GM stuff the clunk went away and never had another problem. Hope this helps.