Understanding The Oil Viscosity Grade Chart

by Synthetic Oil | Last Updated: May 29, 2022

So you want to change your car’s or truck’s oil, but you don’t know which kind of oil is the right one. You pull up an oil viscosity grade chart and end up discombobulated by all the numbers and letters. What does any of that stuff mean? You’re in luck because this article is all about understanding the viscosity grade chart. Let’s not waste another second.

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What is Viscosity?

Viscosity is a fluid’s resistance to flow. This is why choosing the right viscosity of motor oil to lubricate your vehicle’s engine is important. You do not want to use the wrong kind. Viscosity determines the strength of a lubricant’s film, as well as how well it will reduce friction within the engine. Too high viscosity will reduce the overall fuel efficiency of your vehicle.

What are the Different Ways to Designate Viscosity?

At the top of the viscosity grade chart are four designations of viscosity: SAE Crankcase, SAE Gear, AGMA Grade, and ISO VG.

The Society of Automotive Engineers (SAE) has two oil grades that describe either an automotive motor oil, SAE 5W-30, or a gear oil (SAE 75W-90).

International Standards Organization (ISO) is used to grade industrial gear lubricant and hydraulic fluid. The grading system from ISO is a universal grade and is seen as, for example, ISO 100. You can find this anywhere in the world.

Lastly, is the American Gear Manufacturers Association (AGMA), which also grades industrial gear lubricants. You may see it as AGMA 3 or 5.

So when you look at a chart, you can compare the designations between one another. Let’s say you have picked up an SAE 140 gear lubricant. That would be similar to an AGMA 8 or ISO 680 oil.

What is Kinematic Viscosity?

Commonly seen on the left side of a viscosity grade chart, kinematic viscosity is used to describe one thing: a fluid’s visible inclination to flow. In other words, it’s how long you watch water or molasses pour from one cylinder to another. This tendency to flow is written in “centistokes (cSt),” which signifies the relationship of flow volume vs time. For oil, kinematic viscosity is defined as “the resistance to flow and shear due to gravity.”Kinematic viscosity is tested at both 104°F (40°C) and 212°F (100°C).

Now, why is knowing the kinematic viscosity of an oil important? Oils do not all respond the same way during temperature changes. Therefore, you can choose between low and high kinematic viscosity to meet the needs of the engine in various climates.

What is Saybolt Viscosity?

Centistokes may be the more common method of measuring kinematic viscosity, but it can also be defined with Saybolt Universal Records (SUS). While this is becoming a rarer measurement, you can still find many viscosity grade charts with Saybolt viscosity on the right side.

If you want to convert a cSt value to SUS, take the centistokes when the oil was tested at 210°F and multiply the number by 4.664.

What Do Oil Viscosity Numbers Mean?

Now, what about the letters present in SAE grades? Take 5W-30, for instance. The 5W part is the oil’s viscosity at low temperatures. The 30 at the end is the viscosity grade at normal temperatures. The lower the numbers, the lower the viscosity and the faster the rate at which it travels through the engine.

5W-20 provides less resistance than 5W-30, though they have about equal viscosity at start-up in lower temperatures. Compare that to 10W-30, which is a thicker oil and may cause your car to struggle to turn over on a cold day.

A Passing Grade

Hopefully, you now understand what the numbers and columns on the viscosity grading chart mean. You should now be able to compare the various designations and numbers on the chart to find the one that’s right for your project.