Determining proper oil-change intervals has long been a controversial topic. Change the oil too often and you waste time and money and risk polluting the environment. Wait too long and you risk your engine’s good health.

Technology to the rescue: Many cars now come with a system that displays a message on the instrument cluster indicating when it’s time for a change. Rants on the Internet, however, would have you believe that such indicators are the work of the devil—devilish automakers, that is, who program the oil-life monitors to extend change intervals so that engines wear out sooner, forcing motorists to buy new cars.

But you can dismiss such conspiracy thinking. Oil-life monitors do their job quite well, says Ryan Stark, president of Blackstone Laboratories. Since 1985, the company has analyzed hundreds of thousands of oil samples from ordinary folks’ engines. Oil is an engine’s lifeblood, and like a blood test, an engine-oil analysis can identify potential problems before they become serious—and expensive.

Unlike Stark’s lab, though, most oil-monitoring systems don’t actually measure various substances in the oil. Instead, they track driving habits, using algorithms that take into account such variables as mileage, engine speeds, and operating temperature to estimate when the oil is likely to be cruddy enough to require a change.

The monitors typically alert the driver to change the oil between 5,000 and 10,000 miles. But the interval could be shorter or longer depending upon your driving routine. If most of your driving is long-distance at relatively high speeds, the monitor may not indicate a change for 12,000 or even 15,000 miles. If tootling for a few minutes down the road to the market or mall is the only driving you usually do, the monitor may go off in as little as 2,500 or 3,000 miles.

Oil lubricates the engine’s internal moving parts, and short-distance driving is particularly hard on motor oil. Moisture from condensation and combustion gases form acids and sludge in the oil that inhibit lubrication and accelerate wear. Allowing the oil to reach its optimum operating temperature burns off contaminants—and that may take up 20 or 30 minutes of driving, especially in colder weather.

Though oil-life monitoring systems work well, you’ll still need to crack open the good book—your car’s owner’s manual. “And that means reading the fine print,” says Steve Mazor, chief engineer of the Auto Club’s Automotive Research Center. For instance, the manual may say not to exceed 10,000 miles between changes despite the oil-life monitor’s instructions.

Or, it may specify an oil change once a year, even though the car’s oil-life monitor hasn’t called for one. An annual oil change also gives your mechanic a chance to inspect your car and alert you to problems that might have developed during the year.