Two new engine oil blends in development for more than five years have been officially released on the trucking market: CK-4 and FA-4. The oils are the offspring of the Proposed Category 11, or PC-11, engine oil classification.
CK-4, on the other hand, will offer backward compatibility, allowing for use in the vast majority of older heavy-duty diesel engines while still offering increased performance and protection gains. CK-4 oils will replace API CJ-4 engine oils.
According to API, the new CK-4 oils will provide a number of improvements over CJ-4, including improved shear stability, oxidation resistance, and aeration control. API FA-4 oils will provide similar improvements over CJ-4 oils, but it is important to note that FA-4 oils are only intended for use in newer on-highway diesel engines. FA-4 oils will have limited or no backward compatibility with on- and off-highway diesel engines where engine manufacturers recommended CJ-4. While not specifically backward compatible, FA-4 oils are expected to play an important role in some current and new diesel engines by protecting those engines and at the same time helping them meet more stringent emissions requirements.
American Trucker has plenty more on what you need to know about the new oil formulations here.