If you look in the owners manual of almost any car, it says you should change your oil once a year or every 7,500 miles in passenger car and light truck gasoline engines. If you read the fine print it says the recommendation is based under "normal" driving circumstances. Well in any "normal" circumstance you use your vehicle more often than suggested. Meaning you make a lot of short trips, drive on dirt or gravel roads, and drive in a lot of stop and go city traffic.
My suggestion, which a lot of people also suggest, is to actually change your oil in your car every 3,000 miles or 3 to 4 months. That way you are assured that your engine will get its full life.
A new engine can probably get by with changing the oil every 7,500, but as you drive more and the engine accumulates more miles, blowby increases. This dilutes your oil because more unburned fuel gets dumped into the crankcase. So if the oil isn't changed often enough your engine will get more wear and you will get less performance out of your vehicle.
