No experience with Royal Purple, but they have a good reputation. The most important aspect in the tranny is GL4 vs. GL5. Gear oil for the diff has a very different - and mutually exclusive - function than gear oil for the transmission. Everything you want for the diff works against you for the transmission. That's a bit of an oversimplification, but you get my drift. You want the synchros to shear a bit to grab the gear cones, while you DON'T want any such thing with the ring and pinion.

-Bill
'87 4Runner w/ '96 5VZ-FE, 'Red Chili II'
'97 Taco XtraCab 3RZ-FE, 'BlackBean'
TLCA # 13257, Rising Sun 4x4 Club Land Use Coordinator
"He who stops being better stops being good." -Oliver Cromwell