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Engine oil viscosity explained

When choosing an engine oil, there are various factors to consider. One that crops up particularly frequently is viscosity, but what exactly does this mean? More importantly, why does engine oil viscosity matter?

What is viscosity?

The term viscosity refers to the resistance of a fluid – which can be liquid or gas – to flow. Any liquid or gas will display a certain level of resistance to movement within the substance, or a change in its shape.

The opposite of viscosity is fluidity, which measures the exact opposite – how easily a substance flows. In scientific terms, viscosity may be explained as a process of friction between the molecules that make up the liquid or gas. Viscosity is a key factor where fluids are used for lubrication purposes, as the viscosity controls the flow of the liquid over the surfaces of moving parts.

How viscosity affects engine oil

In engine oil terms, viscosity is rated according to how easily the lubricant will pour at a set temperature. Thinner engine oils have a lower viscosity than thicker lubricants, and can be poured more easily at a low temperature. Thinner oils can help to reduce friction within engines, and this can assist with starting up in cold weather conditions.

Two levels of engine oil viscosity

It’s not quite as simple as that, however, as there are potentially two levels of viscosity when it comes to engine oil. It is possible for one lubricant product to have two viscosity ratings; one when the liquid is cold and another when it is hot. This tends to be the case when special additives are present within the oil that prevent it from thinning too much. Engine oil becomes thinner as it heats up and thicker as it cools down, which is why engine oil additives can be so important. These are more likely to be present in top quality oils made by leading manufacturers, as they truly understand how oil affects an engine’s performance.

How engine oil viscosity is displayed

When reading an engine oil’s label, you can see how it should perform in both cold and hot conditions. Engine oil viscosity is denoted according to a scale set by the Society of Automotive Engineers (SAE). The code on the packaging will therefore read along the lines of XW-XX.

The first number before the W denotes the lubricant’s flow at zero degrees Fahrenheit (which is equivalent to -17.8°C). The W then stands for winter. The lower this first number, the more the oil will remain thin in cold weather conditions.

The second part of the sequence, after the XW and dash, indicate the engine oil’s viscosity at 100°C. This number can be used to judge the lubricant’s resistance to thinning when using at high temperatures.

So, an oil labelled 5W-30 would thicken less during cold weather than another lubricant with a 10W-30 rating. This 5W-30 grade oil would also thin more rapidly at high temperatures in comparison to a lubricant displaying a 5W-40 grade label.

Why climate matters when choosing engine oil

Where vehicles are used in colder climates, especially during winter, the engine will work best when a lubricant with a low winter viscosity is used. In hotter climates or during summer, the engine would be most efficient when using oil that displays a higher viscosity at 100°C.

This is why the location in which the vehicle is used can be key when selecting the right engine oil. Thinner oils that will thicken less when used in low temperature conditions can help with starting the engine from cold. Conversely, thicker oils that will thin out less during hot weather can ensure a better engine performance during summer.

0W-20 and 5W-30 oils have thus been formulated, and are recommended, for use in colder climates. At the other end of the scale, 15W-40 and 20W-50 oils have been tailor made to cope with warmer regions.

Why engine oil viscosity matters

When there are low temperatures, engine lubricants should resist excess thickening. This is important as it ensures oil flow to all of the engine’s parts, which is impeded when an oil is too thick. It also matters because an oil that is excessively thick will require more effort from the engine when turning the crankshaft, as it will be partially submerged in thick, viscous oil. Where a lubricant is overly thick, it can be more difficult to start the engine. This also causes more fuel to be used, and has cost implications. This is why a 5W oil will typically be recommended for use in cold conditions.

Synthetic engine oils, however, can be tailor made so that they flow more easily during cold weather. They can thus sometimes pass testing to fulfil the 0W rating. Synthetic oils are generally superior to conventional oils, and the price tag will often reflect this.

When an engine is up and running, the lubricant’s temperature will increase. The second part of an oil’s viscosity rating – such as the 40 in 10W-40 – means that this particular lubricant will remain thicker at higher temperatures than an oil that has a lower second number – like a 10W-30 oil.

The key to choosing engine oil

All of the above may help to explain why engine oil viscosity is important. While this is interesting, and helpful in understanding why the right engine oil is required, there is little need for the average user to know too much about what engine oil viscosity is and how it affects the engine.

This is because what really is crucial in choosing the right engine oil is that you use the correct viscosity as recommended by the vehicle owner’s manual. When the car and its engine was made, the manufacturers would have determined exactly which engine oil would work with that specific vehicle best. They would also have taken into account the market in which the car was sold, and this would include the climate of that country or region.

The first step in choosing engine oil with the correct viscosity is therefore to check the owner’s manual. At that point, a choice between brands can be made, according to budget and preference.

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