Is synthetic motor oil bad for old cars?



A lingering debate around synthetic motor oil is whether it can be used in older cars. In this Engineering explained video, Jason Fenske clarifies things. The video is part of a series sponsored by Mobil1, which gave Fenske access to its engineers for questions about engine oil.

The short answer is yes, you can use synthetic oil in older cars. Synthetic options even exist for classic cars, Fenske said, adding that Porsche has been filling their cars with synthetic oil since 1996. As always, however, it’s best to check the manufacturer’s recommendations, he noted.

Synthetic motor oils have been around since the 1970s, so why do some people think they can’t be used in older cars? Some first synthetic oils could damage the engine gaskets, but the companies have changed their formulations since then, and are specifically testing gasket compatibility, Fenske said.

Oil cap

Another myth about synthetic oils is that they clean the engine more thoroughly, exposing leaks, Fenske said, adding that it’s hard to see why that would be a bad thing. No one wants an engine to lose oil, but if the only prevention is sludge buildup and other buildup, you’ve got bigger problems.

Fenske also discussed the use of thicker oil in older engines. It is generally best to use the weight of oil specified by the manufacturer, but wear can affect the clearances in an engine. In this case, a thicker oil could help, but only as a last resort for a worn out engine. It will also not stop the leaks, as this is related to the faulty seals.

All of these questions relate to older engines, the kind for which manufacturers market high mileage oils. High mileage oils typically have a blowing agent designed to recondition the seals, helping to limit the amount of oil that leaks, Fenske said. They can also be slightly heavier than a comparable standard oil, which helps reduce oil consumption, he said. Ultimately, however, high mileage oil cannot reverse the effects of time, so a worn out engine will always have problems.


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