I thought Amsoil's big selling point is they are not a petroleum based oil. I thought they said they were using PAO base stock?
Jamie I may have misrepresented the product, I should have said in my experience Amsoil is compatible with Petroleum products. From AmSoil FAQ: A. AMSOIL synthetic motor oils are compatible with other conventional and synthetic motor oils. Mixing AMSOIL motor oils with other oils, however, will shorten the oil’s life expectancy and reduce the performance benefits. AMSOIL does not support extended drain intervals where oils have been mixed. Mixing other oils with AMSOIL motor oils may also void the AMSOIL limited warranty. B. 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.
). The group III and group IV's with additives produced today meet and/or exceed what true synthetics did years ago. At one time Amsoil marketed that it was the only true PAO basestock oil company. They had this on all oil bottles, packages, and flyers. Now, like Mobil and others that were once true synthetics they have found they can go with a "better" petroleum base or a petroleum/synthetic blend and still get good protection at low cost.