What does the engine oil specification SAE 15W-40 API CK-4 indicate?

The specification SAE 15W-40 API CK-4 indicates an oil that is particularly suitable for diesel engines. The “SAE 15W-40” part refers to the viscosity of the oil, where “15W” signifies that the oil has a viscosity grade of 15 when cold and behaves like a grade 40 when the engine is at operating temperature. This multi-viscosity characteristic allows the oil to maintain effective lubrication across a variety of temperatures, which is especially beneficial for the high operating temperatures typically found in diesel engines.

The “API CK-4” designation refers to the American Petroleum Institute's classification, specifically indicating that this oil meets the performance requirements for modern diesel engines that use diesel particulate filters (DPFs) and are subject to stringent emissions regulations. Oils that meet this specification provide better protection against oxidation, wear, and sludge formation, making them ideal for the needs of heavy-duty diesel engines.

What differentiates this specification from those suitable for gasoline engines or other types is primarily its formulation designed to withstand the unique demands of diesel fuel combustion and the operational stresses present in diesel machinery. Thus, the specification confirms its appropriateness for diesel engine applications, while other options refer to different engine types or applications that this oil is not