Most of us already know that regularly scheduled oil and filter changes are a vital part of routine car maintenance that is necessary for engine longevity.

During the miles between changes, the oil in your vehicle's engine gradually becomes contaminated with soot, metal particles, gasoline and other impurities. As mileage accumulates, so do the contaminants in the oil. Eventually, enough impurities accumulate that the filter cannot remove them and the oil cannot keep them in suspension.

When this happens, the resulting sludge can dramatically shorten engine life.

Even though modern technology has vastly improved engines and the oils that lubricate them, regularly scheduled oil and filter changes are still a vital part of regular, preventative auto maintenance. The question is: Just how often does the oil need to be changed?

At regular intervals
The old mantra of responsible motorists that said, "Change your oil every 3,000 miles!" is being replaced. Today car manufacturers' recommendations for oil changes range from 5,000 to 7,500, or even 10,000 miles for many cars when operated under "normal" driving conditions—more often if conditions are "severe."

Before you relax, let me point out that the way most of us drive is listed as severe by the automotive experts. According to the owner's manuals of many new cars and trucks, normal operation of a vehicle is at least 20 minutes, at a medium speed with a steady throttle, and in a clean environment.

According to one car maker's owner's manual, the oil change interval is seven months or 7,500 miles under normal driving conditions, three months or 3,000 miles under severe conditions. This is how severe is defined in the manual:

 

• Repeated, short distance driving.
• Extended periods of idling or low speed operation.
• Extensive use of brakes.
• More than 50 percent driving in heavy city traffic during hot weather above 90 degrees F.
• Driving for a prolonged period of time in cold temperatures and/or extremely humid climates.
• Driving in dusty conditions or sandy areas.
• Driving in areas where salt or other corrosive materials are being used.
• Driving on rough or muddy roads.
• Driving in mountainous areas.

Well, it looks like the old mantra about oil changes isn't out of date after all. But what about synthetic oils, don't they extend oil change intervals?

Several car manufacturers do recommend synthetic oil for their high-performance models because of the superior protection from friction and extreme temperatures. The protection comes from the processing required to manufacture the base stock and the different additive packages they require.

Despite this better performance, synthetic additives wear out just as they do in conventional motor oils, and need to be changed. The key is driving conditions — normal or severe.

How about just changing the oil filter, can a car be driven farther between oil changes?

Changing the filter is great, but the oil must be changed along with it, given that the oil's additives wear out and contaminants get into the oil itself. The filter can only capture the bigger particles suspended in the oil, not replenish spent additives.

Pinching pennies
From an economic point of view, it would appear that waiting an extra few thousand miles between changes will save money. For example, based on $25 oil changes every 5,000 miles instead of 3,000 miles, more than $300 is saved per 100,000 miles. Sounds pretty good, but if you drive the average of 17,000 miles a year, it will take you almost six years to reach that mileage — or $50 per year savings.

Extending the oil change interval beyond what the manufacturer recommends can seriously shorten the useful life of your engine, and cost you several times that $300 savings. Just ask a professional service technician how often he or she changes the oil and filter on their personal vehicle. Chances are the answer will be 3,000 miles.

That's the answer given by Don Beyer, automotive services director for AAA's Washington/Inland chapter. Beyer, an ASE master technician and former repair-shop owner, says that it is imperative to change the engine oil and filter every three months or 3,000 miles. "I'm doubtful that anyone, regardless of where they live, drives under the normal conditions defined in owner's manuals."

Bottom line: Don't push the limits of motor oil and flirt with damaging your car's engine, especially when chump change is all it takes for an oil change.

Larry Hall writes about cars and the automotive industry from his home in Olympia, Wash.

Courtesy of MSN.com