The factory stuff is hard to beat. Amsoil is some unreal stuff, I have seen it do things no other lubricant can in industry ASTM testing. Mobil is good, Royal Purple is decent, Redline is junk. For you guys running the Zexel Torsen and Eaton True-Trac, the F-series Superduty factory fill performs really well with these or any other type of mechanical differential not using friction materials (clutches, cones, ect.).
I tried AMSOIL in my Truck, did the Front and Rear Diffs, Tranny, and Transfer Case, it was very hard to shift into 2nd gear with it in there so at my next change I swapped back to good old conventional.