That's an interesting question and a big part is marketing...but then... sometimes you never really know and have to go with what makes you feel better or satisfied. Just this morning my bride and I were discussing our varying gasoline prices here in Miami. If all the same fuel meets the same guidelines, then why is the off-brand, Orion, Premium $3.29 a gallon while the Shell station charges $3.55? And they are about two miles apart... With our bikes the cost difference is less pronounced but when I fill up my 36-gallon truck, that's about $9 more per fill-up.... but for me, I go to Shell. BTW, I've been happy with Amsoil. It also supports my friend, a retired AF Chief who's a dealer.
-MIG