Steve,
I've been using MT-90, simply because I'm afraid to use a GL-5 lube. I'm about to try Amsoil synthetic 75W-90 GL-4. Mind you, I don't expect to feel much of a difference. In the document below, Pennzoil synthetic 75W-90 GL-4 comes out on top but I haven't been able to find it in a quantity less than 5 gallons.