motor oil weight

If you’ve ever enjoyed the satisfaction of changing your own motor oil, you likely purchased motor oil from an auto shop at some point along the way. With a wide array of options when it comes to brand, volume, type of motor oil, and weight, it can be daunting to find the right motor oil for your car. Even if you’re an experienced DIY oil changer, you may be wondering what exactly motor oil weight is! In this article, we break down everything you need to know about motor oil weight so you can select your next motor oil confidently.

What Does Motor Oil "Weight" Refer To?

As you may have surmised, the “weight” of your motor oil has nothing to do with its actual mass. Motor oil weight has more to do with its viscosity or the temperature at which it flows most optimally.

How Is Motor Oil Weight Measured?

The Society of Automotive Engineers (SAE) determines oil weight by heating each oil to 210°F and observing performance. (This is the temperature at which most engines typically run.) Essentially, this test describes a motor oil’s viscosity.

Why Is It important to Select the Right Weight Oil for Your Engine?

If you can imagine the viscosity of molasses relative to water, you know there’s a significant range of viscosities in the natural world. When it comes time to find the right motor oil for your engine, you’ll need to find one with the appropriate viscosity for the job! This ensures moving parts are lubricated effectively with your engine’s longevity in mind. Certain viscosities may be better suited to your driving conditions or climate, in addition to the make and model of your car.

How to Find the Right Oil Weight

With so many different motor oils to choose from today, it can be tricky to find the perfect match for your rig. In this section, we break down how to read the numbers on a motor oil label so you understand what they mean.

1.     Determine if you’re looking for a high or low viscosity oil. This can come from the owner’s manual of your vehicle or by way of recommendation from a local mechanic who knows your climate, driving conditions, and car. A simple rule is, the higher the number on the motor oil label, the higher its viscosity. Need a high viscosity oil? Try SAE-60 or 20W-50.

2.     Know which number on the label refers to oil weight. As you may have noticed from the first point, some motor oil labels contain multiple numbers—so which one refers to the viscosity? In the above example, the number “60” refers to oil weight. This indicates that the Society of Automotive Engineers gave this oil a rating of 60 for viscosity. Since there is only one number on the label, this is referred to as “straight-weight” motor oil. We will break down the second example in the next point.

3.     Know what the “W” means. As is evidenced by the above examples, some motor oil labels have two numbers separated by a hyphen; so, if the second number is the motor weight, what does the “W” refer to? Contrary to popular misconception, the “W” does not refer to “weight,” but “winter.” The “20” in the example is an indicator of the oil’s weight at lower temperatures, or in winter! The “50” in 20W-50 refers to the acting weight of the oil when it is heated to running temperature. Thus, this type of motor oil is referred to as “multi-weight.”

Should I Use Straight Weight or Multi-Weight Oil?

When it comes to straight or multi-weight motor oil, the two are not interchangeable! Check with your owner’s manual or with a local auto shop to see which motor oil is right for your car.

Typically, straight-weight motor oils are used in classic cars or motorcycles, or vehicles not used for all-weather commuting.

Want to Learn More About Motor Oil?

Hopefully, this article has empowered you to read the label on a bottle of motor oil a little more confidently. Depending on where you live, how you drive, and the kind of car you have, selecting a motor oil can mean the difference between mechanical failure and optimized engine performance.

For more information on selecting a motor oil for your car, click here—or browse our current lineup of Amsoil motor oils!

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