If you drive a car, you've probably heard this one piece of advice countless times: Make sure your vehicle gets regular oil changes to preserve the life of your engine.

Regular changes are required because the oil that lubricates and helps to cool your engine has a limited useful life. If it becomes too contaminated with dirt and metal shavings, it'll wear down your vehicle's internal engine parts. Oil can also lose viscosity, or become too thin, with the unpleasant result that it can't properly keep your engine's parts from grinding against one another.

But let's face it, changing your vehicle's oil is messy and inconvenient when you do it yourself. If you're disciplined, you might even go to the trouble of writing down the date and odometer reading of your last oil change, as well as when your vehicle is due for another service.

It's only slightly less inconvenient to bring your car or truck into a quick-lube establishment, having to take time out of your busy schedule. And what if your oil is actually fine and doesn't necessarily need to be changed at a 3,000-mile (4,828-kilometer) interval? In that case, you may be placing an unnecessary burden on the environment by contributing to the pumping, transport, packaging and disposal of those extra quarts of oil.

But on some cars and trucks, there's a device that removes the guesswork from knowing when your oil has outlived its usefulness. The oil life indicator lets a driver know when it's time for a change, based not only on mileage, but on actual conditions that affect the quality of the oil.

Depending on the vehicle manufacturer and the specific equipment used, oil indicators come in two basic varieties: algorithm-based and direct measurement.

Algorithm-based oil indicators measure lots of factors and then plug the resulting numbers into a formula. Based on the answer to this complex, ongoing math problem, the indicator display will tell you whether the oil is OK, is close to requiring replacement or needs replacing immediately.

Interestingly, with these types of indicators, there are no sensors to detect the quality of the oil itself. Instead they combine data on how many miles you've driven, the temperature variations during that time and data about how much work the engine has performed. Typically, the indicator (monitoring system) will receive such data from the powertrain control module, or PCM, which is the main on-board computer. Engineers have figured out a fairly accurate and reliable way to calculate the remaining oil life this way, without having to actually sample the oil.

Direct measurement oil life indicators measure the condition of the oil -- the opposite approach to the system described above. This method uses sensors to sample the oil and determine its remaining life based on any of the following:

  • Conductivity -- how easily electric current passes through the oil (typically, the lower the electrical resistance, the more contaminants are in the oil)
  • Mechanical properties -- piezoelectric sensors can tell how thick the oil is by the force feedback it gives when sloshing around
  • Soot concentration -- dirty oil's days are definitely numbered
  • Presence of water -- water is an impurity in oil, since it hampers the oil's effectiveness and can corrode metal surfaces

Different oil monitoring system manufacturers may use a combination of these measurement techniques. Typically, the information will display as a digital readout on the vehicle's instrument cluster. The display can feature a green, yellow or red-style status bar, with red indicating the "change oil now" zone; it could be a percentage, displaying a text message, something like "40 Percent Oil Life Remaining," or it might just be a light or a message that just comes on automatically when it's time for an oil change.

If your vehicle doesn't have one already, should you be adding an oil-life indicator to the list of must-have options on your next ride? And if you do have one, should you trust its computerized judgment?

It might not hurt. General Motors estimates that drivers of its oil monitor-equipped vehicles could have two to three times fewer oil changes performed each year. Theoretically, according to GM, if all the GM oil monitor-equipped cars on the road observed the maximum interval for changing oil, instead of the oft-advised every 3,000 miles (4,828 kilometers), it could result in 100 million fewer gallons of oil being consumed annually. Nonetheless, GM still advises changing the oil at least once a year, regardless of how few miles you put on the odometer.

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