What Does the W Stand For in 5W-30 Motor Oil?

The viscosity of a fluid is the measure of its internal molecular friction resisting flow. The “W” in 5W-30 motor oil stands for “winter” and indicates the oil meets or exceeds certain criteria for good low-temperature performance. Motor oil with SAE viscosity grade 5W-30 behaves like an SAE 30 oil at high temperatures and SAE 5W oil at low temperatures, providing the necessary fluidity for rapid starts and efficient engine operation at low temperatures.

Without the introduction of additives, as the temperature rises oil viscosity thins. As the temperature drops the oil viscosity thickens. With the introduction of additives, oil can be rated for one viscosity when it’s cold and another when it’s hot. A thicker oil viscosity at higher temperatures usually continues to provide lubrication protection to moving parts. A thinner oil viscosity at lower temperatures flows easier and requires less energy at start up.

For 5W-30, the “5” tells you how the oil will perform at colder temperatures. A “5W” protects and performs better at lower temperatures than a “10W”. The “30” tells you how the oil viscosity will be at higher temperatures. The second number “30” will stay thicker and protect better at higher temperatures than a “20”.

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