Reply from Amsoil dealer. What about it?

[Sparkman]

Castrol's (and now Mobil's, and every other major oil company's) synthetic oil is synthesized from a petroleum base.

Mobil 1 and Amsoil are both group IV, PAO, polyalphaolefin, typically made from ethylene gas. The other over the counter oils that call themselves synthetic are actually group III, hydrocracked petroleum. You are correct that the difference between the performance of a group III and a group IV is narrowing.

Castrol Syntec in the USA is group III, but supposedly the Castrol synthetic that is made in Germany is PAO.