What happens if I use the wrong weight (viscosity) motor oil?

Buying motor oil

People often wonder if it is safe to use a thicker motor oil or a thinner oil than what their motor manufacturer recommends. A common question is whether it is safe to use, for example, 10W-30 oil instead of 5W-30 oil. While it’s best to use what’s recommended in your owner’s manual, using a viscosity one degree higher or lower than recommended generally won’t cause long-term damage.

Let’s look at two engine oil examples to illustrate.

Ruud buys a new car that requires 0W-20 motor oil. He hangs around enough gearboxes to have heard the old axiom that “higher viscosity oil equals better wear protection.”

Wanting the best possible protection for his new ride, he drains the factory 0W-20 and installs 15W-50 race oil.

Next we have John. He is a lover of cost savings, so he buys a 1998 Toyota Corolla. It calls for 5W-30 motor oil. But he has some 10W-30 in his garage, so he uses it to change the oil. No point in wasting good oil.

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Engines are built to use a certain viscosity of engine oil. Today’s advanced engines are built to much closer tolerances than their predecessors. For example, the clearances between the crank journals and the main bearings are tighter. This is done on purpose to allow modern engines to use lower viscosity engine oils such as 0W-20 and even 0W-16.

Lower viscosity oils reduce internal friction because they flow more easily than higher viscosity oils, improving fuel economy. With fuel economy standards tightening, automakers are leaning towards low viscosity lubricants to help them meet the requirements.

Find the right viscosity oil for your engine

Motor oil

Thick engine oil may not flow fast enough

In Ruud’s case, his 15W-50 race oil may be too thick to flow fast enough to fill the spaces between the crank journals and main bearings while the engine is running.

The oil does not form a consistent lubricating film, allowing metal-to-metal contact and wear. His engine was specially designed to use a lower viscosity oil, in this case 0W-20. The lower viscosity allows it to flow faster and fill the small gaps between parts, resulting in a durable, consistent lubrication film.

Not only that, but the engine also wastes energy pumping the thicker oil, reducing fuel consumption. Because thicker oil does not transfer heat as well as thinner oil, operating temperatures will rise, potentially leading to accelerated chemical degradation and harmful sludge and deposits.

Some oil viscosity differences are less pronounced

In John’s case, using 10W-30 instead of 5W-30 reduces potential problems.

His older bike was not built to the same tight tolerances as Ruud’s. In addition, both oils have the same viscosity once the engine has reached operating temperature. He knows this because the second number in the viscosity rating of each oil (i.e. “30”) is the same. It describes the oil’s resistance to flow at 100°C, or normal operating temperature.

However, using 10W-30 instead of 5W-30 can make cold starts more difficult.

It is helpful to think that the “W” stands for “winter”. The lower the “W” viscosity of the oil, the easier it will flow when cold. In this case, 5W-30 will flow more easily at startup than 10W-30.

Some automakers even allow you to switch to a lower viscosity oil depending on the weather.

Find the right viscosity oil for your engine

Thin may not be in. What if John went off the deep end and used 0W-16 in his 1998 Corolla instead of the recommended viscosity?

Just as using an oil with too high a viscosity can lead to problems in Ruud’s engine, using an oil with too low a viscosity can have the same result.

“Oil that is too thin cannot develop a consistent lubricating film, inviting metal-to-metal contact that causes wear.”

Extreme stress and heat add to the challenge. Because oil thins when heated, the already too thin oil becomes even thinner under extreme heat, exacerbating the problem.

Oil that is too thin can also lead to insufficient oil pressure for your vehicle’s variable valve timing system to work properly, if equipped. Low pressure can also prevent lifters from staying in contact with cams, causing noise and increased wear.

Using thick motor oil or thin motor oil

Using a viscosity one degree higher or lower than what is recommended for your engine is unlikely to cause permanent damage.

But to allay any concerns about engine protection and your vehicle warranty, it’s best to use the viscosity recommended in your owner’s manual.