Royal Purple has a loyal following in alot of circles, the OTR truckers, racing enthusiasts, ect. Trans and rear end oil are also regarded as good oils. This a good reputable company.
Go to the analysis section and you will see that many oils have a problem. It depends on the engine, driver, weather and other variables.
I can show you bad analysis on most brand oils. And I can show you outstanding ones on the same oil.
Mobil I is also a good choice. Available OTC at most chain stores.
A couple of guys at work use Royal Purple and claim that their vehicles run smoother and get better gas milage. I've been tempted to try it out. I use Mobil 1 mainly because its easy to get and I have had decent analysis results with the SS versions.