With all due respect, if that truly was the case, every manufacturer on the planet would be insisting their vehicles only used Amsoil, don't you think? With all the complicated, expensive things that are done to gain only a tenth of an mpg here and there, getting +3mpg merely by using Amsoil would be pretty easy and obvious, no?
GM just introduced "Dynamic Skip Fire" on their 5.3L V8 engines...it can shut down any combination of cylinders and involves very complex lifters, oil passageways, electronics, and sensors. RAM has a motorized air dam which extends down at highway speed and back up at slow speeds to improve mpg. Of course, these strategies are only giving minute fuel gains.
I don't believe any manufacturer "recommends" Amsoil, despite the fact that many manufacturers DO recommend some other brand of oil, such as Castrol or Mobil.
Just like Judge Judy says "If it's too good to be true...it probably is."