My personal opinion is tha even if it cost $10 per quart, oil would still be too cheap to leave in an engine for 25K+ miles. Much of it has to do with your driving pattern. If long highway trips are normal, you can go longer distances as the moisture will be evaporated out of your oil by this type of driving. Short trips, and city driving, or dusty conditions mean shorter oil life. What happens to your GM Warranty or extended warranty if you extend oil change reccomendations beyond factory reccomendations?
Amzoil at one time was not certified by SAE, are they now?
To me it just doesn't seem to be worth it to gamble on oil not being contaminated for that long.
For now, I'm sticking with Mobil 1, and 4 - 5K oil change intervals. I have 5 vehicles that I have maintained in this manner, and 3 of them have well over 100K mile with no engine problems, no oil consumption. and rock solid reliability. I'm going to stick with what I know works.