I don't know what is best, but I used Valvoline Transfer Case fluid. I didn't even know that it existed until I was shopping online for XL-12 and gear oils. I ended up buying both the gear oil and transfer case fluid from Advance, and with the 25% discount that was offered at the time, it was all pretty fair priced.
I will be getting a VOA for the Valvoline next week and post it when I get the results. It says for use in Ford XL-12 and GM Auto Trak II applications.
2013 F150 super crew 4x4 5.0 @ 87k+ miles
Edge 5W-20 / Puro Boss PBL22500