Never tried the Royal Purple, I have the GM Syncromesh in mine and it works fine. I don't see a need to try anything else, but RP seems to have good stuff from what i've seen/heard/read although some will disagree.
I researched it before replacing mine a while ago and people seemed to have the best luck with the syncromesh and it has been recommended by Tremic if I remember correctly, but don't quote me on that.