I used to think it would be good to use Royal Purple for my 1991 Mustang GT. And I see that most ppl with muscle cars use it too but a few friends have told me that royal purple is not good oil. I dont remember the reason why but im not sure if its true. So im wondering for my next oil change if I should use that again or try something better. So whats the best oil for my car? And is royal purple a bad oil?