I have used RP in my LR's tranny, transfer case, diffs, etc. I haven't had any problems. But from the studies that are done:
http://www.enhancedsyntheticoil.com/Royal_Purple_vs_AMSOIL.htm It doesn't hold up as well as Amsoil. I only use this in the '04 Rover. It isn't safe to use with real rubber seals/gaskets/etc in the old ones.
I dont want to start a pissing contest but I can tell you this. Any study paid for or sponsored by a company is going to be swayed in that companies favor, period. I'm not saying Amsoil is bad, BUT, if you check the API website, the only oils of theirs API certified are the XL line up which almost no one uses, and it is not what is used in these comparisons.
If you check most car & truck owners manuals, it will say something to the extent of; use only API certified XXX (CI-4, CJ-4, etc...) of an appropriate weight and grade... blah blah blah.
All of Royal Purples oils (not counting the race only ones) are API certifed. The oils Amsoil brags about, are NOT API certified. That being said, its your vehicle to do with as you wish.