What Does 15W-40 Mean? Understanding Engine Oil Viscosity Grades
15W-40 refers to an engine oil’s viscosity grade as defined by the SAE (Society of Automotive Engineers), indicating how the oil performs at different temperatures. The “15W” (W stands for winter) shows the oil’s flow characteristics in cold conditions, meaning it remains fluid enough to protect the engine during cold starts, while “40” represents the oil’s thickness at normal operating temperatures, providing strong lubrication and wear protection when the engine is hot. This makes 15W-40 a versatile, multi-grade oil commonly used in diesel engines and some gasoline engines, especially in vehicles operating under heavy loads, high temperatures, or demanding driving conditions.