The proper viscosity is the single most important criterion of a lubricating oil. Multiweight oils (such as 5W-30) have polymers added to the oil. The polymers allow the oil to have different weights at different temperatures. Multiweight oils perform a great service of not being too thick at cold startup to provide engine wear protection by flowing more quickly to critical engine parts. The first number, the 5W of the 5W-30 viscosity, for example, indicates the viscosity of the oil at a cold temperature; the second number indicates the viscosity at operating temperatures. The lower the first number the better flow ability at startup, especially at cold temperatures.

You should also be aware that multiweight oils are backward compatible. In other words, an 0W-30 can be used with a 5W-30. The second number is important.

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