I used Royal Purple oils for a couple of years.
I switched over from Redline and Neo synthetics for a few reasons. One was price/availability, also Redline was specifically mentioned to be damaging to the center diff of the Subaru AWD system.
The Royal Purple did a great job of quieting a noisy gearbox and keeping my LSDs in working order (actually I think that it tightened them up a bit).
In the engine, it seemed that there was no difference whatsoever between my old Mobil 1 and the new Royal Purple. So I used the purple in the gearbox(es) and Mobile 1 in the engine. It's easier to pick up a quart of Mobil 1 than it is to find a quart of RP/Neo/Redline when you're running low.

A few weeks ago, following a service tech's advice, I changed the engine oil over to Mobil 1 (0W30). I've put on @1000 miles since then. My engine is quieter now, and runs a little cooler. Most importantly, my fuel economy has improved dramatically. My last tank (@85-90% highway at 70+mph) worked out to an astounding 18.7 mpg.

Royal Purple is good stuff, but for the sake of cost/availability/recent performance I'm going fully over to Mobil 1.