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An oil rating, such as 5W-20 and 5W-30, indicates the viscosity of the motor oil. The viscosity rating is determined by the resistance to flow the oil exhibits under a given temperature. Higher viscosity ratings indicate a thicker oil, which therefore flows less easily. Low-viscosity oils are used in certain engines to minimize oil flow resistance and to maximize performance in cold weather conditions, while high-viscosity oils provide increased protection to engine internals, especially under high temperatures.
Many types of motor oil feature two numbers in the viscosity rating, such as in the case of 5W-20. This is because these types of oil feature two different viscosity ratings under different temperature conditions. Multigrade oils feature a mixture of polymers in the oil substance. These polymers increase in size as temperature increases, thus raising the viscosity of the oil. The first number in multigrade oil viscosity ratings is the winter rating, indicated by the initial W. This is the oil viscosity at 40 degrees Celsius. The second number indicates the oil viscosity when heated to 100 degrees Celsius.
5W-30 Versus 5W-20
5W-30 and 5W-20 motor
oil both feature the same winter rating. This means that both grades exhibit the same viscosity under cold conditions. However, 5W-30 features a slightly higher viscosity at 100 degrees Celsius than does 5W-20. This means that once the engine reaches normal operating temperature, 5W-30 oil will be thicker than 5W-20. Due to the increased resistance of the thicker 5W-30 oil, your engine will produce slightly lower fuel economy and horsepower output. You also risk engine damage when using 5W-30 instead of 5W-20, as the internal engine components are specifically designed to be used with 5W-20 motor oil.
Specific information regarding the motor oil suggested by the manufacturer of your vehicle can be found in your owner's manual. For many vehicles, this only includes the oil viscosity rating. Frequently, car manufacturers also suggest a specific type of oil, such as one featuring a special synthetic blend or rating. Additionally, your owner's manual will note if any other oil grades are acceptable. Some manufacturers suggest a slightly lower viscosity rating for very cold climates. If no alternative to 5W-20 is listed, using a different grade of oil may void the powertrain warranty of your vehicle.