Motor Oil Age Doesn’t Matter as Much as Mileage: Study

An oil testing laboratory has found that the conventional wisdom of six-month oil changes might be a waste of your money.

byJames Gilboy|
NEW YORK, NY - NOVEMBER 2: An oil barrel is placed underneath a car during an oil change November 2, 2023 in New York City. The average price for an oil change in New York City using synthetic oil is over $100. (Photo by Robert Nickelsberg/Getty Images)
Robert Nickelsberg via Getty Images


When it comes to caring for cars we cherish, it's better to play it safe than sorry. If you're like me, you've been changing your oil every six months whether you've done 900 miles or 5,000, wondering if you were premature when it doesn't come out inky black. Well, those days are over for me, because oil analysts at Blackstone Laboratories have tested old oil from a variety of sources, and they say it's the miles that matter most.

The results of several tests were outlined on an episode of the company's podcast Slick Talk: Powered by Blackstone Laboratories. The episode in question was uploaded in December, but only recently gained traction on social media such as Reddit. In it, host Joe Adams recounts pulling a half-full bottle of Mobil 1 0W40 off the shelf, and contacting its producer to find out more about it. The bottle turned out to be almost 14 years old, and Mobil 1 said its age meant it'd broken down and wouldn't properly protect an engine. But when pressed, it wouldn't explain how, so Blackstone ran it through a battery of tests.

Test tubes with used motor oil. Blackstone Laboratories

Blackstone said it tested a variety of the oil's characteristics, from measuring additives to water content, flash point, viscosity, insolubles present, and the oil's ability to neutralize potentially harmful acids and bases. Even though the oil was made in September 2010 and had been sitting half-full for an unknown period, it tested just fine. That led to several follow-up tests of other old oils from varying sources to work out the circumstances that cause oil breakdown—and the answer was pretty clear.

Sampling oil from several engines, from a 2020 Ford F-150 with the 3.5-liter Ecoboost with six months on its fill to engines that'd held their fills for five-plus years, Blackstone consistently found low-mileage oils to be within spec. Tests were run not just on gas cars, such as a 1996 Lincoln Town Car that'd gone five years and an unknown number of miles since its last change, but also a diesel 2000 Ford F-350 with the 7.3-liter Powerstroke. There was even a test of 10-year-old oil from a 1995 Porsche 993 that'd done just 776 miles since its last change, and it too measured good.

For the most part, anyway. Some tests revealed higher iron or silicon content, which could indicate wear. But their potential sources are too numerous to say with certainty that the oil was at fault.

"Calendar time is no reason to rule it out," Adams said of old oil. "Miles are what you need to be keeping track of."

Naturally, there are exceptions to rigid mileage schedules. Engines that spend many hours idling or see severe service, like towing heavy loads or track use, should get more frequent changes. Aircraft and other engines with open breathers that can admit water should also see stricter oil change schedules. But for my cars? From here on out, I'm watching the odometer, not throwing away oil that's still the color of whiskey after six months.

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