What Is Oil Viscosity? Oil Viscosity Explained
Most people know that motor oil is vital for an engine’s performance and longevity. But what is oil viscosity? What does oil viscosity mean, and what do those numbers on a bottle of oil mean? This article will answer those questions and more.
The simplest explanation is viscosity measures the resistance of the oil to flow, or how easily oil pours at different temperatures. It’s a range of the thickness or thinness of oil at a certain temperature range. It’s actually the most important physical property of motor oil.
We need oil viscosity to form the lubrication film between metal parts within the engine. It has to be thick enough to form a film keeping engine parts separated but low enough so that it doesn’t cause excessive energy loss. The viscosity of oil varies with temperature changes. A thin oil has a lower viscosity and pours easier in cold weather, while a thicker oil has a higher viscosity and pours more slowly.
For instance, a low-viscosity oil such as an SAE 0W or 5W reduces engine friction and allows engines to start quickly when the weather is cold. A higher viscosity oil like an SAE 40 or 50 does a good job of maintaining film strength at warmer temperatures.
To illustrate, pour out a cup of coffee in the sink. What happens? It flows out of the cup, right? Try doing the same thing with a bottle of shampoo. Shampoo pours more slowly out of the bottle because it’s more viscous than coffee; it has a higher viscosity.
Olive oil is another example. At room temperature, it pours easily. It thickens and pours more slowly if you stick it in the refrigerator for a few hours. That brings us to the viscosity index (VI).
The viscosity index measures how viscosity changes relative to changes in temperature. Temperature affects viscosity. Remember our olive oil example? In other words, VI measures both the ability of an oil to resist becoming thinner at a higher temperature and thicker at a lower temperature.
How do they determine the VI of engine oil? They measure its viscosity across a wide range of temperatures. The higher the VI number, the less change in oil viscosity in relationship to temperature. Motor oil with a high VI protects the engine better. A full synthetic oil generally has a higher viscosity index than conventional motor oil.
What’s the most ideal motor oil? It’s one where the oil’s viscosity would never change at any temperature range.