Understanding the Oil Viscosity Chart: Guide to Selecting the Right Oil

Viscosity refers to how difficult it is for a liquid to flow. It dictates how you select the engine oil for your vehicle since a wrong choice can decrease your vehicle’s performance. 

In other words, understanding how oil viscosity affects your car and which oil is the right one for your car is an important question you should ask yourself before changing your oil for the first time.

That doesn’t mean you have to sit in an hour-long lecture on oil viscosity. Instead, SAE has developed an oil viscosity chart you can use to get started within minutes. 

Read on to learn more about the oil viscosity chart to select the right oil for your vehicle’s engine.

Engine Oil Number Codes 101

The engine oil number code is to engine oils what a name is to humans. 

Arnold Schwarzenegger!

And you just pictured him in your mind. Similarly, SAE 10W-30 means oil of a specific viscosity.

The correlation between the oil’s name and its viscosity was decided by SAE and automotive manufacturers.

Here’s how it breaks down:

Engine oil code consists of two parts:

• Oil viscosity grade when it’s cold — indicated by W
• Oil viscosity grade when it’s hot 

For instance, a multigrade oil SAE 10W-30 means: the engine oil has a viscosity grade of 10 when it’s cold and a viscosity grade of 30 when it’s hot. 

What does the viscosity grade mean?

Viscosity grade is a measure of oil viscosity: the lower it is, the lower the oil viscosity, and vice versa. 

In other words, SAE 0W-40 will flow better than SAE 10W-30 when it’s cold as 0W is less than 10W. But when you’re driving, the SAE 10W-30 will flow better as the oil will get hot and its hot oil viscosity grade of 30 is lower than 40 of the other engine oil.  

On the other hand, SAE 5W-20 will always flow better than SAE 20W-50. 

Reading the Oil Viscosity Chart 

There are different classifications of viscosity grades depending upon the oil type. All of these oil types are displayed in the middle of the viscosity chart. 

Basic Understanding of the Oil Viscosity Chart 

In the oil viscosity chart, you’ve got different viscosity grades in the middle and the corresponding kinematic viscosity and saybolt viscosity at two temperatures on the left and right, respectively. 

Kinematic Viscosity 

Kinematic viscosity refers to the difficulty in the flow of a liquid due to gravity. For example, honey is thicker and heavy. That’s why it flows slowly compared to the light water. 

We have the kinematic viscosity on the left of the chart, and we measure it in Centistokes or cSt. In usual viscosity charts, we can check the kinematic viscosity of an engine oil at warm temperature (40 °C) and hot temperature (100 °C).

Saybolt Viscosity 

Saybolt viscosity is another measure of engine oil viscosity. It’s based on the time it requires in seconds to heat oil. 

You can find the saybolt viscosity on the right of the chart, and we measure it with Saybolt Universal Seconds or SUS. In typical viscosity charts, you can find the saybolt viscosity of an engine oil at warm temperature (100 °F) and hot temperature (210 °F)

ISO/ AGMA/ SAE Viscosities

In the middle of the viscosity chart, you’ll have 4 different oil grade types. While you’d usually deal with SAE oil grades, you can read the viscosity of engine oils of ISO and AGMA oil grades using the viscosity chart.

How Does Oil Viscosity Affect Engine’s Parameters?

Oil viscosity directly impacts the engine’s performance. Let’s see how:

Engine’s Performance 

The oil viscosity grade you choose decides the rate of fuel consumption.

How?

If you run your hand through a bucket of water and a bucket of honey, you’ll see that it’s much easier to move your hand in water than in honey. In other words, you’ll have to apply more force to move an item in a high-viscosity liquid than in a low-viscosity one. 

Similarly, a high viscosity grade oil needs the engine to work harder to perform the same motion. That’s why high viscosity grade engine oils eat up more fuel. 

But that doesn’t mean you should go for the engine oil with the lowest viscosity. Like honey sticks on your finger, a high viscosity grade oil sticks to the engine’s moving parts and builds a thick layer of lubricant. 

This thick layer prevents the moving parts from striking each other, reducing friction and avoiding high temperatures. 

Fuel Economy 

If you want to save on your fuel, go for low-grade viscosity oil. The low-grade viscosity oil provides less resistance to the engine’s parts and even lubrication throughout the engine. 

Emission 

Low-grade viscosity oil contributes fewer emissions. It cuts out on CO2 and dangerous NOx emissions. 

Selecting the Right Oil Viscosity Grade 

To select the right engine, understand the following two things:

• A thin oil flows well and reduces fuel consumption. But at higher temperatures, it may not form the film over the engine’s moving parts you need for reduced friction. In fact, it may even evaporate if your engine overheats. 
• A thick oil doesn’t flow easily and consumes more fuel. But it protects your engine even at higher temperatures. 

Put simply, you need to find a balance between the two extremes and choose an engine oil that’s right for your vehicle. The best way to do that is to start by checking your vehicle’s manual. 

You’ll find the manufacturer’s recommendation for the most effective oil type or range of oil viscosity grade you can opt for. After that, you can choose between engine oil of different brands (or different viscosity grades if there’s a range). 

Final Thoughts: Oil Viscosity Chart — Guide to Selecting the Right Oil 

The engine oil you select decides how your engine performs. A wrong choice can lead to high fuel consumption or an overheated engine. 

Use the engine oil viscosity chart and the vehicle’s manual to always get the right engine oil for your vehicle.  Explore About Engine Oils’s blog for more information on engine oils.