Amsoil has zinc in it. Zinc is great for crankshaft bearings. Zinc creates a glass like coating on metal surfaces when it gets hot. Not many oils contain zinc. That is likely the viscosity difference as there are several formula's for zinc additives in oil.

 

Every engine oil that I know of still contains zinc in the compound Zinc dialkyldithiophosphate (ZDDP)but there's less of it in most oils due to changing API standards.