Amsoil diesel oils are changing in 2017 and AMSOIL synthetic diesel oil’s 5w-30, 0w-40, 5w-40, 15w-40 that exceed API CK-4 and API FA-4 specs have been developed and extensively tested under extreme conditions. With the API introducing two new diesel oil specifications API CK-4 and API FA-4 oil needed to be changed. These 5w-30 0w-40 5w-40 15w-40 CK-4 oil’s will be replacing the CJ-4 specification and this has led to some confusion. In most cases, you will require an oil that meets the new API CK-4 specification. CK-4 is backward compatible with the previous diesel specifications.