How often to change the oil is like asking how often to wash the car.

In reality, you can't wash the car enough. As the dirt and contaminents sit on the surface, they are damaging the paint. If you are really nuts, you'd wash the car every evening. In reality, most people weigh the time and expense and decide that once a week is perfect. If you always garage your car day and night, maybe you can go two weeks or more. It depends on many variables.

Changing the oil is the same way. As the engine runs, the oil is picking up contaminents and combustion by-products. You could change it every week, or every month. Most people decide that every 3,000-5,000 miles is about right.

I live in Southern California and my car is garaged. It is never started in temperatures below 65 degrees (my garage) unless it is already warm. I do a lot of highway driving so the car is always fully warmed up after each trip. I change my oil (synthetic) every 7,500 miles. This is perfect for me (time and expense) and my car.

Oh, I wash the car (it's my wife's) every Saturday.