Motor oil is an essential component to keep your engine running smoothly and efficiently while extending the life of your vehicle. Since so many types of motor oil are available, you want to make sure you choose the right one to protect your vehicle.

Understanding what motor oil does

Motor oil moves through the engine system and its components to lubricate them and pick up dust and other contaminants that can harm the engine. The oil becomes dirty over time and the oil filter becomes clogged with the dust and debris. Both of these components need to be changed periodically, according to the manufacturer’s instructions.

The engine oil must flow through the system and return to the oil pan where it sits when the engine is turned off. Oil is measured by its viscosity. This criterion measures how thick the oil is. An example in the difference in viscosity is comparing honey and vinegar. Honey is thick and flows very slowly when poured. Vinegar is thin and drains out quickly when it is poured.

Oil must function in various temperatures. It must be able to flow in the cold weather of winter while also maintaining the correct thickness in the heat of summer.

Why viscosity matters

If the oil is too thick when the engine is turned on and cold, it won’t be able to flow and protect the components. If the oil thins down too much when the engine is running hot, it won’t lubricate the components adequately. Ensuring that the viscosity of the oil is correct will help reduce wear and tear on the engine while optimizing the efficiency and performance of the engine.

Determining viscosity

The Society of American Engineers, or SAE, has developed a grading system that determines how thick the oil is or the viscosity of it. This system is showcased by a set of numbers: 0, 5, 10, 15, 20, 25, 30, 40, 50 and 60. They let you know how thick the oil is and what temperature it is best suited for. When the letter W is placed behind the numbers, it refers to winter and means the winter temperature for the oil.

In the past, people would put in one grade of oil for fall and winter and another grade for spring and summer to ensure optimal protection for the engine. The same product wouldn’t work for extremes in temperature. However, variations in temperature happen at all times because the engine starts out cold and warms up as it runs. Today, manufacturers have created multi-grade oils that can be used year-round. This eliminates the need for switching out the oil for the change in temperature and ensures proper protection from a cold start to a hot engine. With multi-grade oils, two numbers are listed. The one indicates the viscosity for cold temperatures and the other indicates the viscosity for warm temperatures. An example would look like this: 5W-30.

How viscosity is measured

The oil is placed into a viscometer and measured for how fast or how slow the oil flows. It is tested at 212 degrees Fahrenheit, which is considered the average temperature of a running engine. An oil with a grade of SAE 30 has a viscosity measurement at 212 degrees Fahrenheit. When using the multi-grade oil, it is designed to perform at high and low temperatures, which means that the flow starts at a colder temperature when the engine first starts and continues to flow at that viscosity as it warms up. The benefit for the vehicle is that it has less dry running and less wear.

The faster the oil flows through the device for testing, the lower the number. When the oil is thicker, it will flow slower and be given a higher number. What many people don’t realize is that a 5W-30 and 10W-30 run exactly the same in a warm engine. The difference between the two is how quickly they run when the engine is cold and first started.

Using additives

Multi-grade oils often have viscosity index improvers added to keep the oil from getting too thin when the engine heats up. At the same time, they thin it down enough for it to begin protecting almost instantly at startup. Having these additives improves the engine wear and performance. However, there is a downside. Over time, the additives are depleted, which will leave the oil too thin to protect your engine. That is one reason regular oil changes are so important.

Synthetic oils generally don’t contain additives and can maintain the right thickness or viscosity in various temperatures. However, they are not designed for use in all vehicles. You must check your owner’s manual to find out what type of motor oil is recommended for your vehicle. If you use anything other than what the manufacturer recommends, you risk voiding the warranty and damaging your vehicle.

Signs you are using the wrong viscosity of motor oil

Often, you won’t know that you are using an incorrect motor oil until the damage is done. This may mean you have a leak or engine components wear out and need to be replaced. Instead, it is better to prevent damage by having a professional technician take care of oil changes.

This article originally appeared on as Why Does Motor Oil Viscosity Matter? and was authored by Joyce Morse.

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