The primary difference between FA-4 and CK-4 is the level of high-temperature high-shear, or HTHS, viscosity. FA-4, which is specifically designed for newer vehicles, provides a slightly lower HTHS, which enables an improvement in fuel economy due to lower “viscous drag” from the oil.
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.
