and now for my theory, feel free to express your opinions on it just don't (flame2)
So basicly, here it is, all of a car engine parts rely on oil to maintain movement and to provide cooling. Suggested oil changes range from 3-5k with normal oil and 4-6k with synthetic i believe. That is under normal driving conditions. I, however, drive my car fairly hard, higher rpm, more changes, etc, then and average driver so i would imagine that my oil breaks down a little faster and gets more mucked up in less chassis milage, however my total engine revolutions is much higher then that of a "normal driver" cuz i rev higher for longer etc. So my engine goes through more of a workout then an average engine, and oilchanges are a relativly inexpensive way to help insure better ware on parts over time. Having cleaner oil may help prevent plugged lines or ware on parts from dirt in the oil and likely lubricates better.
A good way to find out the credibility of this is look at race cars, how often do they change their oil? Most likely they change it frequently, the run high rpms for an extened period of time.
well, there ya go, opinons?