What is oil viscosity — and does it really matter?

Oil influences many things in car's engine. It affects the engine's performance, fuel economy, and operating temperature. Given these, the oil we put in our car engines shouldn't be overlooked. 

To ascertain what kind of oil is right for our car engine, we should also learn the viscosity of the vital fluid. 

What is oil viscosity? 

Oil viscosity is the measurement of the fluid's resistance to flow. Viscosity can be related to water and honey. When poured from a container, thinner water flows much faster than thicker honey.

Two numbers define oil viscosity, which are followed by the letter W that stands for winter. This measurement relates to how oil flows in the engine when it's cold, such as at engine start-up. The second number signifies how oil flows at higher or normal engine temperatures (100 degrees Celsius). 

The smaller the number, the better oil will flow, oil maker Castrol said.  Oil with a viscosity measurement of 5W-30 will flow easier than a 10W-30 at start-up temperatures. In contrast, a 10W-30 will flow easier than a 10W-40 at normal engine operating temperatures.

Oil viscosity is important to note as engine oils naturally thicken when they cool and thin when they are heated. Thin, low viscosity oils flow easier, and in turn, protect engine parts at cold temperature. Thick, high viscosity oils are typically better at maintaining strength to protect engines at high temperatures.

Oil viscosity chart

Motor oil viscosity is often reported using the Society of Automotive Engineers (SAE). Castrol and other oil manufacturers offer a range of motor oils in the following viscosity range: 

• 5W-30
• 5W-40
• 10W-40
• 20W-50 

How to check which oil viscosity is required by the car's engine 

As engine oil viscosity differs from car to car, that measurement is available in your car's owner's manual. Alternatively, the service center should know the oil viscosity for your engine, and a quick Google search about your car's engine should also yield the correct answer. 

Importance of motor oil viscosity

Viscosity is the most important property of a lubricant. How it reacts to changes in temperature, pressure or speed determines how well the oil protects your vehicle, as defined by AMS Oil. 

Lubricants with too low of a viscosity for your engine could cause various faults. For example, metal-to-metal contact and wear in the engine is increased, as is oil consumption, which leads to the engine emitting harmful deposits and more frequent oil refills. Too low oil viscosity could also result to leaking engine seals. 

For instance, the you're driving briskly along the highway during summer, the intense heat your engine generates causes the oil to thin. If the oil becomes too thin, it can fail to adequately separate metal components during operation and thus invite wear.

On the other hand, engine oil with too high a viscosity could also hurt the engine by causing the engine to increase its fluid friction that will reduce fuel economy. High oil viscosity could also increase the engine temperature above operating mark, and lessen the engine's ability to start in colder temperatures. 

Photos from Preston Philippines, Castrol, Roy Robles, and Dylan Afuang