time to search
rotella is supposedly the best group III basestock out there (Shell XVHI), and Amsoil supposedly buys its base from Mobil, which ensures its quality.
The differences really have to do with origin, cost of manufcture, etc. In reality, the longevity of the basestocks in normal service, and their actual performance - for, say OCIs under 10k miles is negligible for the most part, though some opinions may differ.
Doa search, youll find a LOT!