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Got oil?: Why engine oil is so important to keep your car running

The question seems to have been around as long as the automobile. How often should a vehicle's engine oil be changed?

The answer, as with most things in life these days, isn't as simple as it used to be.

We'll address the oil change interval question next month, but this month let's discuss the functions of engine oil, the reasons it needs to be changed periodically and proper oil selection for your vehicle.

Oil, of course, serves to lubricate - or coat - engine parts with a thin layer that prevents metal-to-metal contact, thus minimizing friction and wear. A secondary, and perhaps less known, function of oil is to help cool the engine.

Why does oil need to be changed? Well, as the miles roll by, oil can begin to break down as the result of sustained heat. It can also become contaminated with the by-products of fuel combustion. These contaminants include ash, soot and water, which can form varnish and sludge within the engine. Oil contains detergents that hold these contaminants in suspension. If the oil is not changed often enough, this stuff can build up enough to deplete the detergent additives and begin to damage internal engine components.

It is vital that oil of the proper viscosity be used. Viscosity is a measure of oil's thickness, or resistance to flow. As so-called "multigrade" oils are used almost exclusively in modern automobiles, we'll deal only with these. A typical multigrade oil would be labeled as "5W30." The "5W" represents the ability of the oil to flow at low temperatures, as measured by a standardized test; the "30" measures the flow at a higher temperature; higher numbers represent increased resistance to flow, or thickness. Modern engines have tighter tolerances and smaller oil passages specifically designed for relatively thin oils, selected to improve cold-start protection and fuel economy.

Always use an oil viscosity recommended by the manufacturer of your car. Never use more viscous oil, such as 20W50, in a modern engine in the belief that it will provide superior high-temperature protection; it won't. It could lead to serious and catastrophic engine damage.

With regard to particular oil brands, the American Petroleum Institute (API) provides ratings for engine oils. These ratings are shown on oil containers. Unfortunately, most automobile manufacturers do not consider the API ratings to be adequate for modern engines. Manufacturers have lists of approved oils. Ideally, only oils approved by the manufacturer should be used in your car.

Yep, things just aren't simple anymore. As I suggested in an earlier column, find a quality repair shop that specializes in your make of vehicle, and build a relationship. You'll get consistent, expert advice that will help you to maximize the reliability and life expectancy of your car. The increasingly complex world of engine oils is yet another example of the importance of finding a specialist for your car. There's just too much specific knowledge needed these days to trust your car to a general repair shop.

Next month, we'll delve into the question of the proper oil change interval for your car. That, too, gets complicated.

Cliff Greenman operates European Auto Works, 239 W. Evelyn Ave., Mountain View, with John Polstra and Ralph Foglein. Send questions to This email address is being protected from spambots. You need JavaScript enabled to view it. .

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