http://neptune.spacebears.com/cars/s.../oil-life.html

This is a pretty amazing site.
They just got done testing Mobil1 in a 2002 V8 Camaro with an oil analysis every 1000 miles. They left the oil in it until it needed to be changed according to these tests. They did a filter change at 12,000 miles, and finally ended the test at 18,000 miles. All the results are posted.

They are now in the process of testing AMSOIL and will do the same and see how long it lasts.

Looks like the 3000 mile oil change may be a thing of the past.