Motor Oil Viscosity: Everything You Need to Know

Motor oil plays an important role in the performance of combustion-engine vehicles. Available in synthetic, organic and hybrid formulas, it's designed to lubricate the inner walls of combustion engines. After turning the ignition, motor oil will begin pumping through your vehicle's engine where it's able to reduce friction. The thickness of the motor oil, however, will vary depending on its viscosity.

What Is Viscosity?

Viscosity is a measurement of resistance. It represents the amount of resistance a fluid experiences as the fluid flows. A high viscosity indicates a high level of resistance. With a high viscosity, fluids are thick and, therefore, don't flow easily. A low viscosity, on the other hand, indicates a low level of resistance. Fluids with a low viscosity are thinner and more watery. This allows them to flow more easily than their counterparts with a high viscosity.

How Viscosity Affects Motor Oil

Motor oil requires the right viscosity. If the viscosity is too low, the motor oil may prove ineffective at reducing friction. The watery and thin motor oil will fail to protect moving parts from friction-related damage.

If the viscosity of motor oil is too high, it may not travel too slowly through the engine. The thick and sludge-like motor oil will struggle to make its way through the engine's oil passages. As a result, only some parts of the engine will be coated in oil. The parts of the engine that the motor oil didn't reach may then sustain friction-related damage.

Reading Motor Oil Viscosity

How do know the viscosity of motor oil exactly? It's actually included in motor oil grades. The Society of Automotive Engineers (SAE) developed a grading system for motor oil. This grading system features two numbers, each of which indicates the oil's viscosity. 

The first number indicates the motor oil's ability to flow at low temperatures. The second number indicates the motor oil's ability to flow at high temperatures. When these numbers are high, the motor oil is said to have a low viscosity rating.

Some of the most common motor oil grades are 5W-30 and 10W-30. They exhibit similar viscosity at high temperatures. The difference lies in their viscosity at low temperatures. 5W-30 has a higher viscosity rating at low temperatures, meaning it's thicker initially.

The viscosity of motor oil will change depending on its temperature. As it travels through the engine, motor oil will heat up. This heat will make it thinner and more watery, resulting in a lower viscosity rating. Regardless, you can refer to the grade of motor oil to determine its viscosity at low and high temperatures.