Let's admit it at the outset: Engine oil is not sexy. It does not possess the animal magnetism of a Sharon Stone or a Mel Gibson. Unless you are a petroleum scientist or own 10,000 shares of Mobil stock, you probably won't find yourself flushing and panting when someone starts discussing engine oil at a cocktail party.

But you can save yourself a significant amount of money and misery and get a lot more life out of your car by knowing something about this mundane fluid. You should know how often to change your oil. And unless you take your car back to the dealer, who knows what kind of oil it needs, you should be aware of what kind of oil is going into your engine.Using the wrong oil can damage your engine, suggests Mark Fideli, service manager at Philadelphia's Pacifico Ford, and so can failure to change it frequently enough. Those same two missteps also can result in the voiding of your engine warranty.

"People don't seem to realize that something as cheap as an oil change can save a lot of wear and tear - and a lot of money," Fideli observes.

Employing the wrong-weight oil can leave some engine parts oil-starved in the period directly after startup, Fideli notes. This leads to excessive wear and expensive repair bills.

Failure to change your oil at the proper intervals also can lead to engine-damaging oil starvation, adds Norm Hudecki, director of technical services for Valvoline, an engine-oil manufacturer. In this case, the infrequent oil changes promote the buildup of sludge in the crankcase. That sludge buildup, in turn, completely can block the screen on the sump of your engine's oil pump. This blockage shuts off the oil supply to all the parts in your engine. Hudecki points out that if you don't notice the loss in oil pressure caused by that blockage and continue to drive, "you will totally destroy your engine."

Trashing your engine is a very costly business. Even if you are driving an econobox with a small, relatively unsophisticated engine, a factory rebuilt replacement will set you back a couple of thousand dollars. If you are driving a more expensive car with a high-tech engine, the fiscal pain will be unbearable.

A new replacement for Cadillac's advanced Northstar V-8 will cost you about $4,000 installed. A new replacement for the small but very sophisticated four in the Acura Integra GS-R has a price tag of $3,500, not including installation.

A factory-rebuilt replacement for the 4-liter V-8 used in the larger BMWs will set you back a stunning $14,000, and that doesn't include installation.

So, it obviously pays to know a little about unsexy engine oil. The best place to start getting smart about it is in your owner's manual. This invaluable book will tell you what type of oil to use and how often to change it.

In addition to recommending a change interval for normal service (usually 7,500 miles), the manual probably will caution you to change it more frequently if your driving consists mostly of short trips.

"If you are a salesman who puts a thousand miles on your car each week, you can go 7,500 miles between changes without risking engine damage or reducing engine life," says Hudecki. "But if you do a lot of stop-and-go driving, then you ought to change it more often. I would suggest every 3,000 miles or every three months, whichever comes first."

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The reason for changing oil more frequently if you are a short-hopper is that this kind of driving causes a lot of damaging oil contamination.

Let's look now at what constitutes the right oil for your car. This is very important for you to know if you change oil yourself or have it done by someone other than the dealer. Consult your owner's manual for the right type and weight oil required by your engine.

If you have a conventional gasoline engine, the manufacturer probably wants you to use an oil with an American Petroleum Institute (API) quality rating of SH. Introduced recently, SH supercedes SG as the highest grade of oil available. As such, it is rich in detergents and other useful additives. This SH designation will be contained in a starburst on the front of the oil can.

If you have a diesel engine, you will need an oil with a CD designation.

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