Their website
http://www.mobil1.com/index.jsp
says tthat Mobil 2 is the leading synthetic oil for 30 years.
It also talks about the difference between the carbon atoms that form chains versus the syntheticaly formed molecules and how some contaminents can not be removed from petroleum.
I'm not married to Mobil as I have used Castrol and Valvoline on occasion and it seems to work as well as Mobil in everyday use.
I don't think I've ever seen Amsoil in Colorado retail stores.