[KixxPedia] Engine Oil Viscosity Explained

Why is the viscosity of engine oil so important?

The most important thing to consider when choosing an engine oil is viscosity – the thickness or flow of oil. Depending on the viscosity of the engine oil used in a vehicle, different aspects such as driving comfort, fuel efficiency, engine cleanliness and the overall performance of the car can all be affected. Since using an engine oil with a viscosity suited for the operator’s driving habits and external environment can decrease fuel cost and extend the life of the vehicle, choosing the right one is a critical task for any car owner. Here’s some tips to help you choose wisely.

How is engine oil viscosity classified?

To help you understand the meaning behind the mystery codes marked on the labels of engine oils, here’s engine oil grades explained. SAE International, formerly the Society of Automotive Engineers, sets the standard in classifying engine oils. SAE oil grades such as 5W-30 and 10W-40, printed on the oil container give indicators of how the product will perform in your engine.

If the number immediately preceding the letter “W,” which stands for winter, is closer to 0, it means the oil is thinner or has a lower viscosity, thus having a better rate of flow at low temperatures. The number after the letter “W” indicates the oil’s viscosity at high temperatures. The higher this number, the heavier or thicker the viscosity resulting in better engine protection.

Viscosity and viscosity index – what’s the difference?

Viscosity refers to the internal frictional forces that resist the flow of liquid. In other words, viscosity measures a liquid’s thickness. The viscosity index, on the other hand, is a measure of the degree to which viscosity changes with temperature. In general, as the temperature rises, the viscosity decreases and vice versa. When this change in viscosity due to the temperature difference is small, it is said to have a high viscosity index. The higher the viscosity index, the more capable an oil is at providing better startability at low temperatures and maintaining the oil film well at high temperatures. Thus, the viscosity index is considered to be one of the main criteria in determining the quality of engine oil.

What’s the difference between monograde and multi-grade engine oil?

Products marked with only one numerical grade such as SAE 10W, SAE 30 and SAE 40 are called monograde engine oils while products with two numerical grades separated by a dash such as 5W-30 and 15W-40 are called multi-grade engine oils. A multi-grade oil’s viscosity has been formulated to adapt to both low- and high-temperature environments. Accordingly, in countries that experience distinct seasons with varying temperatures, it is more appropriate to use a multi-grade engine oil.

Let’s look at an example by breaking down the characteristics of a 10W-30 engine oil. According to the table, multi-grade oils starting with 10W have a viscosity equivalent to 7,000 mPa.s or less at -25 degrees Celsius with the boundary pumping temperature equivalent to -30 degrees Celsius or below. Oils with SAE viscosity grades ending in 30 have a viscosity between 9.3 and 12.5 m²/s at 100 degrees Celsius with an HTHS viscosity (high temperature high shear) of 2.9 mPa.s or less at 150 degrees Celsius.

This means that the engine oil will be relatively thin at low temperatures to improve startability during cold weather and will not be too thick at high temperatures in order to form a lubricating oil film. Because the viscosity of this oil is adaptable based on temperature, 10W-30 grade oil is suitable for use in places where the temperature difference is significant.

Which engine oil viscosity should you use?

Choosing the right viscosity for the car depends on environmental factors, engine conditions and driving habits among other things, so it is best to refer to your owner’s manual or get advice from a mechanic first. In general, 5W-30, 5W-40, 10W-30 and 10W-40 grade oils are the most commonly used engine oils. However, if you live in a climate with harsh winters or sub-zero temperatures, it is recommended to use a low viscosity engine oil such as 0W-30 or 0W-40.

The larger the difference between the viscosity at low and high temperatures, the wider the temperature range that engine oil can accommodate. However, if the viscosity value at high temperatures is larger than necessary, the vehicle’s efficiency, driving comfort and engine cleanliness may be negatively impacted.

What happens if you use an engine oil grade different from what is recommended in the owner’s manual?

The engines of each car model differ in speed capacity, temperature and load capacity, so the engine oil viscosity required for each type of engine is different. Using an engine oil with a viscosity higher than what is recommended in the manual may cause power loss due to excessive operation. On the contrary, if an engine oil with a viscosity lower than the recommended number is used, a sufficient oil film may not be formed. As a result, there may be wear or scratches inside the engine which could ultimately lead to breakage.

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