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Oil viscosity and how to choose the right grade


Engine oils are essential to a vehicle’s performance and lifespan, and viscosity is an important attribute. Knowing how to determine the right oil viscosity for your vehicle is vital for the engine’s protection.

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Viscosity, or the thickness of oil, is defined as an oil’s “resistance to flow” and is a key factor to consider when choosing an oil. The use of an engine oil that meets the prescribed viscosity is important to provide proper lubrication to the moving parts of the engine. The engine oil must provide a lubricating film in such a way that there is no contact between the moving parts, thus preventing wear. One way of reducing the risk of wear is to use a high-viscosity engine oil. The higher the viscosity, the thicker the lubricating film and the smaller the chance of metal-to-metal contact. However, a lubricating film that is too thick will cause internal fluid friction, resulting in loss of energy and increased fuel consumption.

oil-analysis-introWhat the Viscosity grades tell you

The SAE (Society of Automotive Engineers) established a numerical code to represent the oil’s viscosity at both hot and cold temperatures. 

For example, if you take an SAE 5W-30 oil, the number preceding “W” refers to the oil’s viscosity at 0ºF (-17.8ºC). “W” is often referred to as winter grade. The lower the number before the “W”, the thinner the oil at 0ºF. In other words, an SAE 10W-30 oil will be thicker than SAE 5W-30 at -17.8ºC.

The number “30” in an SAE 5W-30 oil refers to the viscosity of the oil at 100ºC. So an SAE 5W-30 and a SAE 10W-30 will have the same viscosity at 100ºC.

In cold climates, an engine will benefit from using an oil with lower winter “W” viscosity whereas in warmer climates an engine will benefit from a higher viscosity at 100ºC.

When comparing oils, it is important to take into account the location and ambient temperatures in which the vehicle will be used.  SAE 0W-20 and SAE 5W-30 oils have been developed for colder climates while SAE 15W-40 and SAE 20W-50 oils have been developed with hotter climates in mind.

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Viscosity index

The viscosity of the oil changes with the change in engine running temperature, from cold start-up to operating temperature. Apart from the viscosity, you should also take note of the viscosity index (VI) of an oil. The oils viscosity index (VI) is defined as a measure of a fluid's change in viscosity relative to change in temperature. The higher the VI of an oil, the less the change in viscosity with a change in temperature. The VI of an oil aims to ensure optimum viscosity over the varied temperature range from cold start up to engine running temperature. 

So, which is the correct oil grade for my vehicle?

Determining the right viscosity oil is fortunately relatively easy. Original Equipment Manufacturers (OEMs) have put most engine oils through various vigorous tests to ensure that they pass the grade when it comes to engines. Therefore, always check the owner’s manual to see what viscosity oil they recommended. 

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Often, the owner’s manual will specify multiple viscosity options to make provision for varying ambient temperature ranges. Choose the viscosity grade that best represents the ambient temperature range in which the vehicle will be operated.

Understanding the implications of the different viscosities is extremely important for the protection and longevity of a vehicle’s engine. While additional factors such as specifications and drain intervals are important, oil viscosity should be one of your top priorities when it comes to fleet maintenance.