While you are doing it why not use synthetic. Then you will not have to do it again!.
I have 75w90 Amsoil Series 2000 in mine (since 3k miles).
I tried to do the Transfer case too, but the filler plug is almost impossible to get at.
Um, RedLine 75w90 *is* synthetic , and us racers have a religious devotion to the stuff, though honestly I've never seen a comparison that shows one top shelf synth (RedLine, Amsoil, Royal Purple, Mobil1, Neo) better than any other.
Just got done, and the car feels great! I can totally feel the extra HP and my MPG went up too!!!...