What Does Oil Weight Really Mean?

There are a lot of terminologies thrown around when it comes to oil, which can easily lead to confusion about different products. Oil weight in particular is a bit counterintuitive, as it does not actually describe how much the oil actually weighs if it was put on a scale.

Rather, oil weight describes the viscosity of an oil. Viscosity represents how well the oil flows at a specific temperature. To picture this, you can think of the difference between how cooking oil flows right after you put it on a pan versus after it’s had time to heat up. The heated oil moves much more easily as it is less viscous.

In general, oil’s viscosity will decrease as it heats up.

If you’ve recently purchased a new vehicle or just an engine, you might notice that the oil weight could be lower than what you were previously used to. Many manufacturers are trying to make their engines more efficient, and using thinner oils can allow the rotating parts of an engine to run more easily. The difference it makes is small, but it can still help meet fuel economy standards.

Standardization of Oil Weight

The Society of Automotive Engineers developed the oil weight scale most motor oils use. Generally, the oil is put through tests at around 212 degrees Fahrenheit when measuring the viscosity to determine the oil weight. Since engineers design engines to work with specific oil weights, getting the correct oil for your engine is very important.

You also might want to think of the conditions your vehicle is driving in when purchasing your oil. Many oils have multiple numbers listed for their oil weight, such as 10W-30. The ‘W’ here just stands for “winter”, which you can take to mean cold temperatures.

Since a lower rating means the oil will be less viscous, you might be wondering why the winter rating is usually lower. It’s because the oil has been developed to behave as a lower weight when cold and a higher weight when warm.

Engineers don’t go through the trouble of creating a special oil mixture with carefully chosen additives just for fun. When starting your engine on a cold day, the oil needs to quickly move through everything to prevent parts from becoming damaged. That’s why the winter rating is lower, a higher viscosity oil just wouldn’t move quickly enough.

As your engine warms up your oil will too, and it will eventually reach the normal oil rating.

Choosing the Right Oil for You

Since different working conditions can require different oil ratings, it’s important to read through your engine’s manual to select the optimal product for your use case.

For example, some manufacturers might recommend different oil weights if you do a lot of towing and hauling or if you’re in an extremely cold environment.

Depending on the type of engine you’re using you might need an oil with only one weight listed, such as SAE50 oil.

You definitely do not want to guess when it comes to the oil weight for your engine.

As always, if you have any questions concerning oil weights or anything else related to oil selection, contact AMSOIL, and we’ll be happy to discuss your options with you.