If you compare Amsoil to Mobil 1, product for product, the Noack Volatility of the Amsoil line is significantly lower - as are the Pour Points. This indicates not only that Amsoil is using PAO's, but likely some of the newer, advanced PAOs' such as the EOM, "Spectrasyn Plus" (for their SAE 0w-30/0w-40), and/or the Chevron "Do-Decene" PAO's in the formulation of their top tier lubes. The latest Amsoil MC oils and Four Stroke Marine oils look to me to be "Dumbbell blends" of regular PAO's and the Spectrsyn Ultra PAO's. This allows you to generate very stable synlubes with VI's in the 140-160 range using no polymeric thickeners Given that Amsoil is recommended greatly extended service intervals for all their lubricants and not just their gas/diesel engine oils, I'm not sure they have the luxury to substitute less capable basestocks. For example, their commercial compressor fluids are recommended for drains of 8000+ hours, with periodic oil analysis - that's a full year of running 24/7 between changes. Furthermore, it is difficult enough to make a 25k/1 yr lubricant using the absolute best additives and basestocks available, much less trying to make them with Group III or even GTL stocks. I haven't seen anything that would suggest that Amsoil is using any Group III stock in any products except their XL line and perhaps their 15w-40/PCO, synthetic blend. Attempting to draw conclusions without any hard data (like a Gas Chromotography Scan), is mere speculation and does not even rise to the level of an educated guess.