Can't Fix an Engine Using Thicker Oil

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Dear Car Talk:

I have a 1991 Ford F150 with 183,000 miles on it. I've been using brand-name conventional oil in it since day one. So far, I'm not having any oil-related issues, but when I buy oil, I see specially blended oils for "high-mileage engines," meaning engines with 80,000 miles or more on them.

Is there any real benefit to using special high-mileage oils, or is it safe to continue with a quality, conventional oil that meets the latest Society of Automotive Engineers (SAE) standards? -- Steven



Generally speaking, most people do not need a high-mileage oil. In the old days, when your car was burning oil, people would suggest that you add really thick molasseslike oil to the engine, like some 20W-50 weight glop. The idea was that the thicker oil wouldn't flow as easily, and therefore would have a harder time seeping past worn-out valve guides and valve guide seals and getting into the cylinders, where it was getting burned.

But -- hello! -- you want your oil to flow easily. Its job is to flow and splash, so it covers all of your moving parts and protects them. Plus, it rarely worked. A lot of oil-burning takes place because an engine's piston rings are worn out, and thicker oil won't fix that. Using thicker oil is also a particularly bad solution for modern cars. Not that you're driving a particularly modern car, Steven, but let's flatter it and call it modern for the sake of discussion.

Modern engines rely on the exact opposite of that 20W-50 glop. They use low-viscosity oils that splash easily all over the moving parts of the engine to make sure they're all lubricated. They also count on low-viscosity oils to reduce friction and drag (the work of simply moving the engine parts through thicker oil), which improves mileage. If you look at the specs for new cars these days, they're not calling for 20W-50 oils. They're not even calling for 10W-30 oils. They use 0W-20 synthetic oils. That means it acts like a zero-weight oil when the engine is cold and a 20-weight oil when the engine is hot.

Some "high-mileage oils" now include an additive that's supposed to help soften up hardened, leaky engine seals. So if one of your seals is leaking, you can try a high-mileage oil. But I wouldn't expect it to perform miracles -- any more than Geritol is going to suddenly get your great-grandfather back on the uneven parallel bars. Otherwise, I'd do exactly what you're doing, and use the oil that the manufacturer recommends and that meets the current SAE standards. Or even consider a synthetic.


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