What API oil classification is suitable for engines fueled by ultra low sulfur (ULS) fuel?

The API oil classification that is suitable for engines fueled by ultra low sulfur (ULS) fuel is CJ-4. This classification was specifically designed to address the needs of modern diesel engines that operate under more stringent emissions standards. It provides better control of soot and helps to enhance the engine's performance alongside reducing emissions.

CJ-4 oils are formulated to work effectively with the advanced technologies employed in engines designed to burn ULS fuel, such as exhaust after-treatment systems, which can be sensitive to certain oil formulations. Using CJ-4 oil ensures compliance with manufacturers’ recommendations for emissions control, contributing to overall engine longevity and efficiency.

In contrast, other classifications like CI-4, CF-2, and CH-4 do not meet the requirements set for engines using ULS fuel, as they were developed prior to the establishment of the CJ-4 standard and lack the necessary performance characteristics for modern emission systems. Therefore, while these older classifications might still function in older engines, they are not optimal choices for the latest engine designs that utilize ULS fuel.