I have always used Castrol regular and synthetic on my cars and now with the motorcycles I want to try it also.I used to use Motul 7100, now I use Rotella T6 full synthetic 15-40 which has the Jaso rating and is safe for a wet clutch. I can't tell the difference....except when I pay for it. Rotella is half the price.
But Rotella T6 is not designed for motorcycles, does it?There is no "best". All opinions are subjective. Any oil that meets the manufactures minimum specifications will work. Some of us prefer Rotella T6 for the price.