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You hear the word all the time when you are getting your oil changed, but exactly what is viscosity? It’s what the numbers and letters stand for on motor oil bottles — 10W-30 for example.
Viscosity measures how much the motor oil can resist flow. Imagine pouring motor oil out of the bottle; how fast it comes out indicates the viscosity. The more viscous, the slower it will move. Apple juice has low viscosity, cold honey has high viscosity.
So what do the numbers mean? The number preceding the W designates the viscosity at a low temperature, and the number following represents viscosity when the engine is hot. For example, a 10W-30 motor oil means the viscosity is at 10W when the engine is cold and 30 when the engine is hot.
Low viscosities are good for cold temperatures. Oil naturally “thickens” with cold, but a low viscosity, thinner motor oil flows more easily and moves quickly. When you start a cold engine, motor oil needs to travel to the top of the engine before it circulates back down. Since motor oil is vital to lubricating your engine, it needs to get on the job quickly, and low viscosity helps it do just that when it’s cold.
High viscosities work better when it’s hot. Oil thins with heat, of course, but if it thins too much it won’t be able to lubricate properly, which it does by keeping parts separated by a thin film of oil. Too thin, and metal parts may contact and wear.
So your motor oil is always maintaining a delicate balance: it needs to flow well when the engine is cold but also retain enough body at higher temperatures to keep metal parts lubricated and separated.
Engines that operate in a narrow range (in lawnmowers or the like) may be just fine with a single-grade oil, such as a 30W. Most vehicle manufacturers specify multi-grade oils, though, both because they envision operation in a wide climate range and because efficient, emissions-controlled engines run at high internal temperatures. To find out what motor oil grade is right to keep your engine running as designed, check your vehicle manufacturer’s recommendations or simply Find Your Jiffy Lube®.