5W-20, 10W-30 ... What Do Motor Oil Weights Mean?

Stephen Fogel
July 9, 2018

engine oil weight

When it’s time for an oil change — or just to top it up — you’ll see a few different options, a combination of numbers and letters. But what’s the difference between 5W-20, 5W-30, 10W-30 and so on? 

Your engine’s oil prevents friction and wear while carrying away heat and contaminants. In other words, it keeps the engine healthy. But it has to do this in some pretty extreme situations. These conditions can range from frigid winter mornings to high-speed cruising down an Arizona interstate on a 110-degree day

This is where different grades of oil come in. Let’s take a look at what oil viscosity is and how that translates into the numbers and letters you see on a container of oil. 

What is oil viscosity?

The primary reason why oil is used as a lubricant is its ability to flow through your engine and maintain a coating on the engine’s moving parts. This prevents metal-to-metal contact, which would quickly destroy those parts. Oil’s ability to flow at a given temperature is called viscosity.

Viscosity can be objectively tested by timing how long it takes for a specific amount of oil to flow through a specific-diameter opening at a set temperature. Oils with higher viscosity (a larger number) take longer to flow through the opening, while oils with lower viscosity (a smaller number) flow quicker. Based on this test, oils receive a viscosity number. A higher number denotes a thicker oil.

» LEARN MORE: How often should you change your oil?

Before the 1950s, single-grade engine oils were used. Drivers would use a lighter weight during the summer and a heavier one during the winter. But as cars evolved, got more powerful and ran hotter, single-grade oils were no longer adequate.

To meet these new demands, the oil industry came out with multi-grade oils. Now motor oil could be designed to flow easily at low temperatures, while remaining thick enough at high temperatures. These multi-grade oils have steadily evolved since then to meet the needs of today’s more advanced engines.

What the numbers mean

The numbers and letters on a container of multi-grade motor oil will tell you at a glance what its operating range is and how it compares to other oils. Here’s an example:


5: This is the oil’s viscosity rating at low or winter temperatures

W: This stands for “winter,” and is part of the “5” rating

20: This is the oil’s viscosity rating at high temperatures

What this means is that the oil industry scientists, in their labs, have created lubricants that flow better at low temperatures than they do at high temperatures. This is exactly what your engine needs. You get easy cold starting, combined with excellent high-temperature protection. 

How the numbers compare with each other

Comparing the first, lower “W” number: When looking at two oils with different ratings, start by looking at the first number — the one paired with the “W,” which gives you its cold weather performance rating. In the above example, that would be “5W.” 

A lower number will protect well in colder temperatures. So, 0W will do better in the freezing cold than 10W.

Comparing the second, higher number: Now look at the second number, which gives you the oil’s high-temperature performance rating. In the above example, that would be “20.” 

A higher number will do better in hotter temperatures. So, 40 would be able to handle triple-digit heat better than 20. 

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How to know what motor oil weight to use in your car

If this sounds complicated, don’t fret. There’s an easy way to pick the right oil to put into your vehicle: Check your owner’s manual.

Look up “oil” in the index, or check the reference section in the back of the manual. There you should find the oil the manufacturer recommends you use. Sometimes there will be a choice of oils, based on different climate conditions. Select the oil that matches the temperature range where you live and drive.

If you need to top the oil up but don’t know what type is in there currently, go with the recommendation in the manual — but plan on getting the oil changed soon. This way, if the oils don’t match, the different additives won't be working against each other. But the bottom line is that a full oil pan is far more important than the type or brand of oil used.

While it might not matter as much if you use “high-mileage” oil, “truck” oil or any specially marketed types, you definitely shouldn’t use motorcycle oil in a car or truck. Motorcycle engines are built differently and require different additives that can be bad for your car. if you have any more specific questions about the correct oil to use in your vehicle, ask your mechanic.

Stephen Fogel

About the Author

Stephen has been an automotive enthusiast since childhood, owning some of his vehicles for as long as 40 years, and has raced open-wheel formula cars. He follows and writes about the global automotive industry, with an eye on the latest vehicle technologies.

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