What does 10W30 engine oil mean; let’s talk oil viscosity

If you have ever been to the aisle of your local auto parts store I am sure you have spent a few minutes wondering, why are there so many different types of oil for sale? Lets learn more about oil viscosity.

Well, there are many differences in the oil that is sold on the shelf, there are different oil specifications that are meant as well as different oil viscosities. I am going to teach you all there is to know about the differences in oil viscosity. 

Oil viscosity basically refers to how easy it is to pour an oil at varying temperatures. High oil viscosities do not flow as easily as a low oil viscosity.  

If we rewind the clock nearly 70 years, purchasing oil was much simpler. If I had a vehicle that required 30 weight oil I would go to the store and purchase it. 

In the 1950s lubrication engineers invented a multi weight oil. The only place where you may find a single viscosity oil still today is a small air cooled engine that is located in a warm place. Think of a lawn mower located in sunny California for example. 

A multi weight oil basically means that it has multiple oil viscosity values. To make the oil viscosity standardized and properly tested the Society of Engineers (SAE) created a measurement for engine and transmission oils. 

The standard format that they come up with and you will see on store shelves is 10W-30. We are going to use this oil as our example as we work through its true meaning. Let’s take a look at our chart and than break it down into two sections


What does the 10W mean in engine oil?

The W stands for winter, and the 10 is the viscosity of the oil tested during the winter test. The winter rating is the oil’s ability to flow in cold temperatures. The test is completed at 0°F (-17.8°C). 

The higher the number the harder it is for the oil to flow at this temperature. So for example a 10W oil flows easier than a 15W oil.

This test is completed with a scientific instrument known as a Cold Crank Simulator Test. It is a simple test where an object is moved through the oil that is being tested. The machine tests and measures the amount of energy that is required to complete this. 

What does the 30 mean in engine oil?

It is the ability of the oil’s resistance to thinning at high temperatures, 100°C to be exact. This is the oil’s kinematic viscosity. I know this is going to sound technical, it is the resistance to shearing and flow caused by gravity. 

The higher the value the greater the oil’s resistance to thinning. So a 40 weight oil will resist thinning more than a 30 weight oil. 

This test is completed by allowing the oil to flow through a small orifice and measuring the amount of time required. Think of pouring a glass of water compared to a glass of honey. I haven’t done this experiment but knowledge tells us the honey would flow slower. This would indicate that the honey has a higher viscosity. 

I hope you understand a little bit more of what these numbers now mean. With today’s modern vehicles,  always remember when purchasing oil for your vehicle you buy what the manufacturer recommends.

I know an engineer has spent a lot of time and energy coming up with the perfect oil for my engine, so I will follow his recommendations. Check out our amazing article on service repair manuals.

John Morris
John Morrishttps://autoknowit.com
John Morris is the technical editor for AutoKnowIt.com. His years of experience in automotive repair as well as an automotive professor have prepared him to ensure that even the most technical information is accurate and concise at all times.

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