Oil breaks down the older it gets and loses its ability to lubricate. It may even break down and begin to turn to sludge. Below are a few examples of what old oil can do to an engine. A 3000 mile interval was the standard in the old days, but newer oils can go longer (but I personally don't trust any of them past 5k or so). Guys with lab coats who never work on cars claim that their products can now protect engines for 10k or even 20k miles, but reality is that many newer cars are significantly burning oil after 70k or 80k miles and dealers shrug their shoulders when customer complain and reply that this is normal and acceptable. This didn't used to be the case. Some theorize that manufacturers planned this so they can sell new cars or expensive repair jobs. That sounds a little far-fetched; I think a more likely explanation is that the Marketing department started listening to the lab coat geeks and decided they could claim their cars now require less maintenance. If you sell your car after 50k miles, I guess it's not your concern, but if you plan to keep it a long time, oil changes are cheap insurance.