What Does 15W40 Mean – Understanding Oil Viscosity Ratings

15W40 refers to a multi-grade engine oil viscosity rating defined by the SAE (Society of Automotive Engineers). The "15W" indicates the oil’s viscosity in cold temperatures, where "W" stands for winter, meaning it flows like a 15-weight oil when cold to ensure easy engine startup and proper lubrication. The "40" represents the oil’s viscosity at high operating temperatures, showing it behaves like a 40-weight oil when the engine is hot, providing adequate protection against wear and maintaining stability under heat. This dual-grade characteristic makes 15W40 suitable for a wide range of temperatures, offering reliable performance in both cold starts and high-temperature conditions.