I run Amsoil Severe Gear 75/90W.
Solved the severe whine in my Diff where I thought the Diff was going to go bad...
I also use their Limited Slip Additive, and it works well.
PS - I WOULD NOT run just any old oil in our Diffs. Our Diffs are pretty fragile due to size as it is, so anything you can do to help out with better oil is going to help.
Preference from what I've seen myself and read, is Amsoil, seems to prevent shear best and maintain it's properties over time much better over time than others.
Gear Lube White Paper from 2007
I'm sure there is some updates since this was done, and Amsoil Haters will come out of the woodwork, but it's not just a ruse. They do make solid products, just not cheap...
EDIT: I also just remembered that they changed the weight specs a while back and some locol folks switched to the 75/110 Weight with more preferring that for it's viscosity ratings