Those Oil Life Indicators Are Way More Accurate Than You Think

Yes, that oil life percentage remaining on your dash is accurate. And it uses far more than mileage to calculate the number.

engine oil monitoring system percentage

Photo by: YouTube

Brian Silvestro

By: Brian Silvestro

May 11, 2025 at 3:00pm ET

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Many modern cars have an oil life indicator to tell owners when they should have their oil changed. Some are even fancy enough to display a percentage that conveys exactly how much life is left in the oil. If you've ever wondered if that percentage is accurate, well, it is. And it's not just calculated based on mileage.

The first oil life monitoring systems came into the world back in the 1980s, through an algorithm developed by three General Motors engineers: Donald Smolenski, Paul Harvath, and Shirley Schwartz. Using data from the car's OnStar system, like engine cycles, injector activations, and water temperature, the trio created a way for the ECU to predict the wear on an engine's oil.

The algorithm is smart enough to consider heat cycles, factoring in shorter trips and longer trips. And because the system can understand oil temperatures, it also considers how oxidation—a cause of oil degradation—could shorten the oil's lifespan, and factor that into the percentage remaining.

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Any good fuel system will display a bright orange light well before your tank is truly empty. Similarly, GM's system will advise the driver to change their oil before it's totally cooked. It'll also tell you to change the oil at least once a year, regardless of how much you've driven—the bare minimum for oil changes.

In a real-world test, Lake Speed Jr. of The Motor Oil Geek YouTube channel grabs some oil from a Cadillac to confirm the accuracy of its monitoring system. Unsurprisingly, he finds the algorithm a bit conservative, discovering the oil has a bit more life in it than what the system claims. Like a low fuel light, it's just a warning you should get your oil changed, not that you're running on dead oil.

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