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What Does Engine Oil Viscosity Mean

Updated on January 23, 2018

Here’s the thing with engine oils. If you happen to ask a question about it to ten well-informed guys, you are likely to hear twelve answers, all different from each other. Viscosity, by definition, is the measure of a fluid’s resistance to flow. When it comes to engine oils, it indicates the thickness of the oil at a given temperature. Recommended by motorbike manufacturers, viscosity grades are usually printed on your user’s manual and look similar to 10W-20, 15W-40, 20W-50 and so on.

Breaking down the 10W-20 and the 15W-40...

To understand these configurations and the numbers, let’s assume the format they are written resembles: ‘xx-W-yy’, where xx and yy are numbers. Here, the ‘xx’ stands for the oil viscosity level at lower (cold) temperature whereas the ‘yy’ stands for the viscosity at higher (hot) operating temperatures. The ‘W’ stands for winter and not weight, as assumed by many.

What is this high-low?

The basic function of a bike engine oil is to reduce friction, avoid corrosion or wear and tear of the moving parts. To do so, the engine oil needs to be thin as well as thick, thus of the required specific configuration. But why thin as well as thick? At colder temperatures, such as when you’d start the bike, the oil needs to be ‘thin enough’ to reach and cover all the corners of the engine to reduce friction and avoid corrosion as they directly affect the performance and fuel efficiency. For the times when your bike engine is burning hot from long and stressful city commutes, thinner oils would burn away quickly. Thus, you’d need your bike engine oil to be ‘thick enough’ to withstand the heat and continue to function.

Just tell me the best one already...

Waiting for us to tell you the best engine oil for your bike? Here you go. It depends. It depends on factors such as your bike, your riding style, your commute route and so on. The most efficient engine oil would be the one with lower first number (thus thinner) providing easy flow of oil at cold engine temperatures along with a higher second number (thus thicker) to remain ‘thick enough’ to protect the engine at burning temperatures.

Let me use an example to make it easier. Since long, I’ve been using Castrol Power1 engine oil for both my bikes but with different Viscosity. My Royal Enfield Himalayan is topped up with Castrol POWER1 20W-50 whereas for my KTM Duke 390, I decided to go with Castrol Power1 15W-50. The viscosity for both the bikes at the high temperature is 50, which is great. The Duke has comparatively lower viscosity of 15 for better performance at colder engine temperatures whereas the Himalayan’s is 20. Talking from experience, the reason why I choose Castrol Power1 is the wide range of grades and that of the additives it contains. I’ve experienced smoother acceleration, owing to the the additives that keep the engine free from any slug deposits and better fuel efficiency that makes it great value for money. I haven’t seen any other engine oil come close. Whatever you choose to go with, make sure you do not go above the first numbers (xx) and below in the second set (yy) as they would not comply with the minimum grade requirements of your motorbike.


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