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Synthetic oil vs. petroleum oil

Oil isn’t just plain old oil anymore

You’ve just bought a new tractor and the dealer recommends using a synthetic oil. However, it’s more expensive than petroleum oil. What's the difference?

Jim Wills is a biosystems engineering professor at the University of Tennessee. He says crude oil from an oil well has to go through a refining process. Chemical additives are put in to improve engine function. Synthetic oils are created in a lab.  

"It's a much cleaner oil, and it's also formulated for the specific purpose of lubricating an engine and cooling, and sealing, and doing all the things that normal oil is expected to do inside an engine," says Willls. "And so for that reason, the synthetic oil is a much more perfect lubricant than natural petroleum-based oils."

Synthetic oils have basically the same additives as the petroleum-based products. But they also have a higher proportion of additives, such as detergent. Because the oil stays cleaner, you can go longer between oil changes. Synthetics can also withstand both higher and lower temperatures than petroleum oils. However, Wills says avoid using synthetic oil in diesel engines.

"Diesel engines operate under much higher pressures than the gas engine. And synthetic oil may be too thin to lubricate. And when that film layer gets too thin and you get metal-to-metal contact with moving parts, then you're going to get excessive wear," says Wills.

Wills says synthetic oils should not be used in older engines that have heavy sludge deposits around the seals of internal engine parts. The detergent additives might actually clean up the sludge – and cause the engine to leak or consume excess oil.

Learn more about the differences between conventional and synthetic oils

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