Here is a link to a great life study of Mobile 1 vs. Amsoil
http://neptune.spacebears.com/cars/stories/oil-life.html
It is very comprehensive and seems to have been well researched and documented. My understanding of the highlights:
1) Most of the engine wear occurs in the first 3000 miles after a change...this was a big surprise.
2) Both oils seem to be good for well over 10,000 miles. Mobile 1 went 18,000 miles.
3) Amsoil has a severe viscosity increase that actaully affected fuel economy.
4) Amsoil seems to work slightly better in the engine wear department.
I would be very interested to see other opinions oof this test.