What oil does
The honey-hued liquid that practically glows in sunlight as you decant it into an engine is a highly refined version of the same black goo that’s so difficult to wash off seabirds. We usually think of it as keeping engine parts slick and separated enough from one another to minimize wear.
Modern motor oil, with its package of additives, also reduces deposits within an engine, carries off contaminants, inhibits corrosion, and helps cool the engine. But such chores take their toll. Motor oil’s initial beauty and its useful life are fleeting.
How important is it?
Most car engines depend on three liquids: gasoline, oil, coolant. Of these, gasoline is the least subtle: Without it, the engine simply won’t run.
Coolant is a bit more insidious. You can get well under way with nothing in the radiator. When engine destruction looms on the immediate horizon, a dashboard gauge or warning light probably will give all but the most preoccupied drivers timely notice. Even damage from the prolonged use of worn-out coolant often can be reversed, given enough money and time in the shop.
Oil is the most subtle of the three. While operating an engine with no oil causes two rapid-fire events (it activates a warning light or gauge on the dash and follows up immediately with engine destruction), operating with contaminated oil achieves step two in covert slow motion, usually bypassing step one.
Why change it?
The two main reasons are that oil gets contaminated and the additives wear out. Even a partial roster of the misfortunes that befall engine oil is enough to horrify the more sensitive automotive engineers and should at least give warning even to callous drivers: Chemicals resulting from burning fuel worm their way in. Oxygen mixes with it, creating acids. Dirt and other solid matter infiltrate. Gasoline and water dilute it. Additives lose their punch. Viscosity decreases. Sludge forms.
Use the viscosity and service classification recommended by your car’s owner manual. Both are listed on oil containers. Any name-brand will do.
The service classification is given in a two-letter code, such as SH. "S" indicates the oil is for gasoline-powered engines. The second letter indicates the progression of oil formulation changes over the years—the farther along in the alphabet, the more recent the formulation. So far, we’ve advanced as far as "H."
Viscosity is a measure of a liquid’s resistance to flow; the higher the number, the thicker the liquid. For example, blood would have a higher viscosity number than water. A typical oil viscosity number is 30, although multi-viscosity oils have a number that expresses a range, such as 10W-30 ("W" indicates suitability for winter use).
The recommended interval between oil changes is one of the few instances where the car’s owner manual may not be the engine’s best friend. Many manuals recommend oil changes at greater intervals than AAA technicians consider best.
Intervals commonly recommended by manuals are 7,500 miles for normal driving and 3,000 miles for "severe service" (or similar wording). In this case, "severe service" is what most people think of as normal driving. One AAA technician describes the only kind of driving that’s not "severe service" as "when you slowly drive a couple of miles to a freeway, go 30 or more miles on the freeway, then slowly drive a mile or so off the freeway, coasting to a stop." For other types of driving, 3,000 miles between changes is a good bet.
Time is important, too. Try to have the oil changed every three months, even though your car may not have gone 3,000 miles. Short-trip driving is hard on oil. Get a new filter with each change.
If your car has a diesel engine, get an oil specially formulated for diesels. Cars with turbochargers are another special case. Turbos create a lot of heat; they’re lubricated by engine oil and show it no mercy. If your car has a turbocharger, get oil marked "turbo" or "turbo formula."
This article was first published in January 1999. Some facts may have aged gracelessly. Please call ahead to verify information.