The W stands for winter and marks the oil’s cold-start viscosity within the SAE multi-grade system. In 0W-20 and 5W-30, the numbers describe how the oil behaves when the engine is cold and when it is hot, respectively.
What the W stands for
The "W" designation is a legacy from winter-grade testing that shows an oil’s ability to be pumped and lubricate an engine at low temperatures. Oils earning a W rating must meet specific low-temperature pumping-viscosity requirements so a car can start and be lubricated quickly in cold weather. The number before the W is about cold-temperature performance, not weight or height, and it helps determine how easily the oil will flow when the engine is cold.
Winter testing and the meaning of W
The W designation is part of the SAE J300 viscosity classification and is applied to multi-grade oils. It does not refer to weight. It signals the oil’s suitability for cold starts and its ability to circulate before the engine reaches operating temperature.
How to read the numbers
The two numbers in a multi-grade oil label tell you how the oil behaves across temperature ranges. The first number, immediately before the W, is the cold-temperature performance; the second number, after the dash, is the oil's viscosity at 100°C (engine operating temperature).
Cold-weather (the first number) and hot-temperature (the second number) implications
Lower first numbers (like 0W or 5W) indicate the oil remains thinner when the engine is cold, enabling easier starting and faster lubrication during cold starts. Higher first numbers indicate thicker oil at cold temperatures and slower flow; these are less ideal in very cold climates but can be beneficial in hotter engines or climates that require thicker lubrication when hot.
Higher second numbers (20, 30, 40) indicate higher viscosity at 100°C, meaning the oil is thicker when the engine is hot. This provides more film strength and protection under high loads or hot operating conditions, but can slightly affect fuel economy if the engine tolerances allow for thinner oil.
In practice, the right balance depends on your vehicle’s design and the environment in which you drive. Manufacturer specifications will determine the recommended grade for your car.
Here are representative contrasts between common grades used in everyday vehicles:
- 0W-20: Flows very well when cold, supporting quick lubrication on cold starts and helping improve fuel economy; designed for modern engines with tight tolerances and often used in newer vehicles.
- 5W-30: Slightly thicker at cold start than 0W-20 but offers robust protection at typical engine temperatures; widely used in a broad range of vehicles and climates.
- Note: Regardless of grade, you should choose the oil that meets your vehicle manufacturer's specifications.
In practice, the right choice depends on climate, engine design, and manufacturer recommendations; always refer to your owner's manual or the oil specification label under the hood.
Additional considerations when selecting an oil grade:
- Manufacturer specifications: Use the grade recommended by the vehicle manual or dealer; using the wrong grade can affect protection and efficiency.
- Climate: Colder climates may benefit from lower cold-start ratings (0W, 0W-20), while hotter climates or high-load conditions may benefit from higher hot-end ratings (30, 40).
- Engine age and condition: Older engines might benefit from slightly thicker oils for wear protection, but must still meet the engine’s spec.
- Viscosity tolerance: Some manufactures allow a range (e.g., 5W-30 or 10W-30) for certain conditions; consult the manual.
Bottom line: Always check the owner's manual or official lubricant specification for your vehicle to ensure you’re using the correct grade and specification.
Summary
The W in 0W-20 and 5W-30 signifies winter, indicating the oil’s cold-start viscosity characteristics. The first number (0 or 5) describes cold-temperature flow, with lower numbers meaning the oil flows more readily when cold. The second number (20 or 30) describes viscosity at operating temperature, with higher numbers indicating a thicker oil when hot. The best choice depends on your car’s requirements, climate, and driving conditions, and it should always align with the manufacturer’s specifications as defined by SAE J300.