Here are some links to an independant oil study of Mobil one Vs Amzoil and others.
I've been using Amzoil for just under a couple of years and a big fan of same. Draw your own conclusions.

Read this one first
http://neptune.spacebears.com/cars/stories/oil-life.html

read this 2nd. http://neptune.spacebears.com:80/cars/stories/oil-life.html


read this 3rd http://neptune.spacebears.com/cars/stories/amsoil.html