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Motor Oil Viscosity

Omkar Phatak
Viscosity is the most important parameter that needs to be checked while buying motor oil for your car engine's lubrication needs. Here is a short guide on choosing one that suits your car.
The choice of motor oil lubricant for your car engine is very important as it largely affects its performance. A car engine that is lubricated with the right grade of oil is more efficient as its functioning parts work more smoothly. Engine power is not wasted in overcoming friction and you get a better mileage from your car.

Motor Oil Viscosity Explained

Viscosity is the property of every fluid which decides how smoothly and easily it will flow and how much resistance will it offer to its own flow. Thinner and low-viscosity fluids move faster and more smoothly than thicker, high-viscosity fluids. For example honey, due to its high viscosity moves slower than water which has lower viscosity.
It depends on the internal properties of a fluid, including its chemical composition and temperature. With higher temperature, viscosity of most petroleum-based oils decreases and they thin out. So, viscosity of motor oil, being a function of temperature, is never specified without the actual value of temperature, at which it is supposed to offer best lubrication service.
A car engine's moving parts are constantly bathed in oil which forms a thin film on their surface to prevent friction. Another function served by the oil is that it effectively suspends the small contaminants that could cause friction and makes sure that they do not come in contact with moving parts. To maintain consistent lubrication levels, the viscosity needs to be stable over a range of operating temperatures.
The car engine's temperature is never constant. It changes during operation of the engine and also changes with the surrounding temperature. With lowering of temperature, the oil becomes sluggish as its viscosity increases and its lubrication function is less efficiently executed. At high temperatures, during high RPM operation, the engine oil can thin out due to lowering of viscosity and its lubrication film also thins out.
A single grade motor oil is designed to perform efficiently and hold a specific viscosity, only for a certain temperature range. That is why, when temperature changes drastically, a single grade oil's vulnerability is exposed.
In car manufacturer's lingo, viscosity of oil is also referred to as weight. Today, multi-grade or multi-weight engine oils have been developed which can withstand high, as well as low temperatures. This is made possible by the polymer additives in these oils that prevent thinning up of the oil at high temperatures and thickening at low temperatures.

Viscosity Rating

The viscosity of an oil type is decided by the 'Society Of Automotive Engineers (SAE)' and graded by an SAE number. The number decides how fast or how sluggishly oil flows under specified temperature conditions.
The higher the SAE number, higher will be the viscosity of the oil used. For Example, SAE 20 is less viscous than SAE 60. There are special 'Winter Grade' oils that maintain a low viscosity, even at low temperatures.
Multi-grade oils are specified with dual SAE numbers and with a 'W' in between that specifies its winter grade nature. So if a motor oil has a SAE number of 10 W-30, that means it shows a viscosity of 10 at low temperatures that occur in winter, while the second number- '30' signifies the higher viscosity it demonstrates at high temperatures.

How to Choose the Right Oil?

Every car engine is unique and the choice of motor oil for it, is determined by the weather conditions in which it is used. Your car maintenance manual specifies the viscosity value that would be right for your car model. So, open the manual to know its required grade of oil.
Manufacturers provide viscosity charts that can be referred to while buying. If you do not have a car manual, then a general rule to follow is, go for high SAE number oils if you are using your car in colder weathers and a low SAE number motor oil if you live in temperate regions. However, it is best to find out the recommended specifications for your car model and go ahead accordingly.
In summary, viscosity decides how well your engine will perform under varying temperature conditions. Periodic changes of oil are necessary due to the creation of sludge, which is created by suspension of contaminant particles in it. Refer to your owner's manual for recommended oil change timing.