Motor vehicles use multigrade viscosity oil because it can perform well in a wide range of temperatures. In cold temperatures, a low viscosity oil is needed for the engine to start easily and for the oil to circulate quickly to lubricate moving parts. In hot temperatures, a higher viscosity oil is needed to prevent the oil from thinning out and losing its lubricating properties.
A viscosity rating, such as 5W30, is a way to measure the oil's thickness or resistance to flow. The first number, 5 in this case, represents the oil's viscosity rating at low temperatures (usually measured at -25°C or -13°F). The "W" stands for "Winter" and it indicates that the oil has been tested at low temperatures. The second number, 30, represents the oil's viscosity rating at high temperatures (usually measured at 100°C or 212°F). The higher the second number, the thicker the oil will be at high temperatures. Therefore, 5W30 oil has a low viscosity at low temperatures, making it easier to start the engine, but it has a higher viscosity at high temperatures, providing better protection for the engine.