The "W" in oil specifications, such as 5W-30 or 10W-40, stands for Winter. This letter designates the oil's viscosity (thickness) at low temperatures. The number preceding the "W" indicates the oil's viscosity grade at cold temperatures. A lower number means the oil flows more easily when cold, crucial for easier engine starting in winter. This is important because cold oil is thicker and takes longer to reach all parts of the engine, potentially causing wear. Thinner oil flows more easily at low temperatures ensuring proper lubrication even during cold starts.
Several sources confirm this meaning:
- Upkeep.com: Explicitly states that "The “W” that you see on oil labels stands for the word "Winter" because this portion of the label shows the viscosity level of oil when an engine is cold."
- BobIsTheOilGuy: Reinforces the "W" meaning "winter," highlighting that it relates to the oil's low-temperature pumpability.
- US Lube: Explains that the SAE (Society of Automotive Engineers) has six winter classes indicated by a number and the letter "W" standing for "Winter".
- TotalEnergies: Further clarifies that the number before the "W" represents the winter viscosity grade, impacting the engine's ability to start even in cold weather.
- Multiple other forums and websites reiterate this definition.
It is crucial to note that the "W" does not denote weight, despite common misconception. The numbers represent viscosity grades, a measure of an oil's resistance to flow. The number after the "W" signifies the viscosity at higher temperatures.
Examples:
- 5W-30: Flows easily at low temperatures (5W), and has a viscosity of 30 at higher temperatures.
- 10W-40: Flows slightly less easily at low temperatures (10W), and has a higher viscosity of 40 at higher temperatures.
