What Do Oil Grades Really Mean for Your Engine?

Whether you own a car or a motorcycle, one of the most important maintenance items is changing your oil. Compared to replacing a clutch or a timing belt, changing your engine oil is relatively straight-forward. However, not only does modern motor oil come in many different varieties, there are a lot of oil grades available. And the labels don’t always do the best job of explaining everything. However, once you know what to look for, finding the right oil grade and type doesn’t have to stressful.

Engine oil grade vs oil weight

Although ‘oil grade’ and ‘oil weight’ are often used interchangeably, they’re not technically the same thing. But both are tied to the same thing: viscosity, which describes a fluid’s resistance to flow. A low-viscosity fluid, like tap water, flows easier than a high-viscosity one like honey. In the same vein, a low-weight motor oil pours and flows easier than a high-weight one.

However, viscosity isn’t constant; it’s affected by temperature and, to a lesser extent, pressure. As the temperature drops, engine oil becomes syrupy and harder to pump, AutoZone explains. This was especially problematic in older, carbureted vehicles, which needed to be cranked at high speeds to fire up.

But, although viscosity drops as the oil heats up, that’s not always a good thing, NAPA reports. Low-weight motor oils’ films aren’t thick enough to lubricate the engine’s components as well as a higher weight oil. Which means more friction, which can lead to extensive damage. Plus, the thinner oil doesn’t pick up debris as well and can squeeze past seals more easily. So not only do you have less lubrication, you may be burning your engine oil, too.

It’s here where the difference in ‘oil grade’ and ‘oil weight’ becomes more apparent. And it’s also why the engine oil you buy at the store has those 2 numbers on it.

Motor oil grade terminology

Samples of motor oil displayed on a table with Russian labels
TATARSTAN, RUSSIA – OCTOBER 11, 2018: Samples of motor oils on display at the TANECO oil refining and petrochemical complex in the city of Nizhnekamsk, Republic of Tatarstan, Russia | Alexander RyuminTASS via Getty Images

If you look on a bottle of motor oil, you’ll usually see a label structured like ‘XW-X’, with the X’s being numbers. That ‘W’ doesn’t stand for ‘weight,’ though. Instead, it stands for ‘winter,’ Valvoline explains.

In the past, Blackstone Laboratories explains, all motor oil was ‘straight weight.’ I.e., its viscosity wasn’t modified after the oil was produced. Now, though, almost every oil is a ‘multi-weight’ oil, with additives that modify its viscosity.

That’s where the ‘winter’ part comes in, Advance Auto Parts explains. A 5W-30 oil, for example, flows just as well as a 10W-30 oil once the engine’s heated up. But at freezing temperatures, the former acts like a 5-weight oil, and the latter like a 10-weight. Which means the 5W-30 will be easier to pump.

It doesn’t matter if you’re shopping for car or motorcycle engine oil, synthetic or conventional, this terminology holds. And the automotive industry has numerous standards and standard tests set up to measure an oil’s lubricating properties.

Can I use a different oil than my owner’s manual specifies?

The best motor oil grade, The Drive reports, is the one your manufacturer recommends for your vehicle. But does it matter if instead of 10W-40, you add 5W-30? And what about ones for motorcycles?

Modern motorcycle oils have many of the same additives as car oils, Motorcyclist reports. In addition to viscosity modifiers, there are anti-wear and anti-foam compounds, antioxidants, and cleaning detergents, Road & Track reports. However, because motorcycle engines spin faster and share oil with the transmission, they require different oil additives. So even if your bike and your car both use 5W-30 oil, they’re not interchangeable.

On the subject of changing oil grades, though, the answer is, “It depends.” In situations involving extreme temperature and wear, some manufacturers do have alternative oil grade recommendations.

For example, for track-driving a 2019 Corvette ZR1, Chevrolet recommends going from a 0W-40 oil to a 15W-50 one. That’s because, on the racetrack, the engine’s getting hotter and seeing more aggressive driving than on the street, and needs the extra wear protection. And for extremely cold climates, some OEMs recommend going to a lighter-viscosity oil, Amsoil reports.


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However, there are limits to this. Putting a 0W-20 oil in the ZR1 would accelerate engine wear because the oil isn’t providing enough protection. Going from a 0W-40 to a 5W-40 would have the opposite effect: the engine would have trouble pumping it. So theoretically, you can go from a 5W-40 to a 0W-40 or a 5W-40 to a 5W-50. But unless your vehicle’s manufacturer allows it, stick to the oil grade listed in your owner’s manual.

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