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Engine oil viscosity refers to how easily oil pours at a specified temperature. Thin oils have lower viscosity and pour more easily at low temperatures than thicker oils that have a higher viscosity. Thin oils reduce friction in engines and help engines start quickly during cold weather. Thick oils are better at maintaining film strength and oil pressure at high temperatures and loads.
The Society of Automotive Engineers developed a scale for both engine (motor oil grades) and transmission oils.
Viscosity is notated using the common classification “XW-XX”. The number preceding the “W” (winter) rates the oil’s flow (viscosity) at zero degrees Fahrenheit (-17.8 degrees Celsius). The lower the number, the less the oil thickens in cold weather.
The numbers after the “XW” indicate viscosity at 100 degrees Celsius and represent the oil’s resistance to thinning at high temperatures.
For instance, oil with a 5W-30 grade thickens less than oil with a 10W-30 grade in cold weather. Oil with a 5W-30 grade thins out more quickly at high temperatures as compared to oils with a 5W-40 grade.
During winter and for cars used in cooler regions, your engine will benefit from using oil with low winter viscosity. During summer and in hotter regions, your engine will benefit more from oil with higher viscosity at 100 degrees Celsius.
When comparing oils, it is important to take into account the location in which the car will be used. Thin oils that are less prone to thickening in low temperatures will help you start your engine more quickly in winter while thick oils that are less prone to thinning in hot temperatures will help your engine perform better in summer. As a result, 0W-20 and 5W-30 oils have been developed for colder climates while 15W-40 and 20W-50 oils have been developed with hotter climates in mind.