Car companies have been messing with this. They've got enough $$ to waste a bunch of engines to figure it out.

More & more cars calculate oil drain times from more than just mileage. BMW started doing that in the '80s. My '01 Saab just uses miles and months. The new '03 Saab uses all kinds of stuff like calendar time, engine running time, # of revolutions (not miles), engine temperature, & number of engine starts. Dashboard computer can display remaining oil life in %%. People have said it can go as long as 20k. (For it to be valid you gotta use synthetic.) I'm too lazy to calculate all that stuff by hand...

I imagine US-EPA has even been pressuring carmakers for longer oil drain intervals, because they know lots of people still dump used oil on the ground... If not, there's some marketing pressure to have longer intervals, too.

If you really want to get into this, you can have used oil tested. Figure out a good interval for your car, your driving habits. But it won't be the same for other people & different cars.