Basestock is refined petroleum oil with no additives.
There are several chemical companies in the US that make AMSOIL's base oils. Exxon Chemical, Texaco Chemical, Chevron Chemical and others.
Do you mean Mobil 1 5W-30 (which is synthetic) or Mobil Super 1000 (which looks like a HVI mineral oil basestock). The only Mobil 100 5W-30 I remember was a "red band" mil spec mineral oil (and that was 20 years ago).