From Amsoil's page. It eludes to the fact that base stocks may be whatever is necessary, PAO, ester, mineral oils, hydro cracked, etc.
" AMSOIL maintains formulation details as proprietary and does not divulge specifics regarding the type of synthetic base stocks used in its synthetic lubricants. AMSOIL developed the world?s first API-qualified synthetic motor oil in 1972 and has remained the leader in the synthetic lubricant industry by continually researching new technologies and demanding only the highest-quality raw materials. As the company moves forward with new technologies it is increasingly more important that this information remains proprietary. AMSOIL views synthetic base oils the same as it views additives, with each having its own set of unique properties. AMSOIL does not insist on a particular type of base stock, but insists on particular performance parameters. AMSOIL chooses whichever synthetic base stock or combination of base stocks delivers the desired result and tailors its lubricants to be application-specific (gasoline, diesel, racing, transmission, gear, extended drain, extreme temperatures, etc.). At the end of the day, the type of base stock used to formulate the oil is inconsequential; the product?s performance is what matters. "