Most vehicle manufacturers recommend changing the oil once a year or every 7,500 miles in passenger car and light truck gasoline engines. For diesel engines and turbocharged gasoline engines, the usual recommendation is every 3,000 miles or six months.

If you read the fine print, however, you'll discover that the once a year, 7,500 mile oil change is for vehicles that are driven under ideal circumstances. What most of us think of as "normal" driving is actually "severe service" driving. This includes frequent short trips (less than 10 miles, especially during cold weather), stop-and-go city traffic driving, driving in dusty conditions (gravel roads, etc.), and driving at sustained highway speeds during hot weather. For this type of driving, which is actually "severe service" driving, the recommendation is to change the oil every 3,000 miles or six months.

For maximum protection, most oil companies say to change the oil every 3,000 miles or three to six months regardless of what type of driving you do.

A new engine with little or no wear can probably get by on 7,500 mile oil changes. But as an engine accumulates miles, blowby increases. This dumps more unburned fuel into the crankcase which dilutes the oil. This causes the oil to break down. So if the oil isn't changed often enough, you can end up with accelerated wear and all the engine problems that come with it (loss of performance and fuel economy, and increased emissions and oil consumption).

Regular oil changes for preventative maintenance are cheap insurance against engine wear, and will always save you money in the long run. It's very uncommon to see an engine that has been well maintained with regular oil changes develop major bearing, ring, cam or valve problems under 100,000 miles.

WHAT ABOUT THE OIL FILTER?
The oil filters on most engines today have been downsized to save weight, cost and space. The "standard" quart-sized filter that was once common on most engines has been replaced by a pint-sized (or smaller) filter. You don't have to be a rocket scientist to figure out that a smaller filter has less total filtering capacity. Even so, the little filters should be adequate for a 3,000 mile oil change intervals -- but may run out of capacity long before a second oil change at 6,000 or 15,000 miles.

Replacing the oil filter every time the oil is changed, therefore, is highly recommended.