" ... but an off the shelf syn. base stock with some amsoil specific additives."
That's all any synlube is ... It's a matter of how you put them together and how much R&D and QC/QA input you have.
But, my bad. What I should have said is, all Amsoil lubes meet or (greatly) exceed API specs. Last time I looked, only one Amsoil engine oil is API certified. Do you really believe the American Petroleum Institute is going to wrap its arms around and give a big hug to a full-line of extended-drain, 100% synth oils? Nice Dreams.
As I've disclaimed soooo many times, I'm not an Amsoil apostle -- I've just been using their synlubes and greases for 16 years and know with absolute certainty they deliver as-claimed. If you don't want to use them, don't. I'll lose no sleep over that.