80W90 synths seem hard to find (plenty of conventionals though). Amsoil AGL is the only 80W90 synth that comes to mind. I'm using Amsoil Severe Gear 75W110 right now (Amsoil lists it as an alternative to an 80W90 spec). I did use their AGL 80W90 for years. I can't tell/feel any difference between the two. The Severe Gear has the best UOA's that I've seen out of any gear oils I've seen tested.
What are most people here using in their rear diffs?