If you've read the engine oil bible, there's a section in there that talks about how many miles is the "best" distance between oil changes. It's not really about the miles, but more about the conditions. In general, if you live in the mid latitudes where the weather is temperate, two oil changes a year should be what you're looking for. One in the spring, and one just before winter sets in.
Why do I say this?
The life of the oil in your engine is not solely dependent on the number of miles driven. It's more complex than that and includes these factors:
Number of cold starts (more condensation in a cold engine)
Ambient temperature (how long before warm enough to stop serious condensation)
Effectiveness of crank case scavenging
State of wear of the engine (piston blow-by multiplies the problem)
Accuracy of carburation during warm-up period (extra gook produced)
Distance travelled
An average family car will do around 14,000 miles per year and about 2/3 of that will fall in the March - November period. All that distance will have been done at reasonable temperatures, including long distance runs during vacations and good weather. During the November to March period it may accumulate only 2 or 3 thousand miles, all low temperature starts and mostly short runs.
Hence why I say to do two oil changes a year - one after the winter months, and one after the summer months. I find this to be better advice than saying 'do it every x thousand miles' because if you own a car that does 5 miles a day on your commute, it could take a couple of years to get to 'x', meaning you've gone through two hot and two cold seasons without an oil change. Generally speaking, that's not the best thing for your engine.
