SAE 15W-40 is a type of motor oil that is classified by the Society of Automotive Engineers (SAE). The "15W" indicates its viscosity rating at low temperatures, meaning it flows like a 15-weight oil in cold conditions. The "40" signifies its viscosity at high temperatures, allowing it to perform like a 40-weight oil when the engine is hot. This oil is commonly used in diesel engines and some gasoline engines. This multi-grade oil provides good protection and performance across a range of temperatures, making it suitable for various driving conditions. It helps reduce engine wear, improve fuel efficiency, and maintain engine cleanliness. Many vehicle manufacturers recommend SAE 15W-40 for specific models, especially in heavy-duty applications.