If we compare a 5W-30 and a 10W-30, they will behave very similarly as the engine heats up, but at start-up, the 5W-30 will provide less resistance and start easier than the 10W-30. Less resistance results in improved fuel economy but the oil is not as thick and forms a smaller layer of protection between metal surfaces. But as the engine heats up, the 5W-20 will move with less resistance than an 5W-30. Again, the lower the number, the lower the viscosity of the oil and the faster the oil will move around the engine.Īs an example, let’s compare 5W-20, 5W-30 and 10W-30 motor oils. A 5W-20 and 5W-30 will have very similar if not equal viscosity at lower start-up temperatures. In the 5W-30 example, the 30 defines the viscosity of the oil at normal engine temperatures. The high temperature viscosity is the number after the dash and is related to the viscosity of the oil as it is moving around your engine after the car has warmed up and is at normal engine temperature. The lower the number, the faster the oil will flow at vehicle start up. For example, if the oil is a 5W-30, the 5W part describes the viscosity of the oil at low temperatures. That value has the letter “W” after the number and has a dash after the W. The low temperature viscosity of the oil is a measurement that simulates starting a car on a cold winter day. But as motor oil technology advanced, additives such as viscosity index improvers allowed for the use of the same grade of oil year-round. Years ago, most vehicles used one viscosity grade oil in the summer and a different viscosity grade oil in the winter. The measurement of viscosity at high and low temperatures are properties of multi-grade oils. Motor oil viscosity grades are based on a scale developed by the API (American Petroleum Institute) lubricant organization. The values are defined in a specification known as API 1509 and are based on the resistance the oil gives to flowing at two different temperatures – cold and high temperature. Viscosity is the resistance to flow of a fluid.

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