The current diesel engine service categories are API CK-4, CJ-4, CI-4 PLUS, CI-4, CH-4, and FA-4. The previous service categories such as API CC or CD are obsolete. API solved problems with API CI-4 by creating a separate API CI-4 PLUS category that contains some additional requirements – this marking is located in the lower portion of the API Service Symbol "Donut".

API CK-4 and FA-4 have been introduced for 2017 model American engines.[13] API CK-4 is backward compatible that means API CK-4 oils are assumed to provide superior performance to oils made to previous categories and could be used without problems in all previous model engines (but see Ford below).

API FA-4 oils are different (that is why API decided to start a new group in addition to API Sx and API Cx). API FA-4 oils are formulated for enhanced fuel economy (presented as reduced greenhouse gas emission). To achieve that, they are SAE xW-30 oils blended to a high temperature high shear viscosity from 2.9 cP to 3.2 cP. They are not suitable for all engines thus their use depends on the decision of each engine manufacturer. They cannot be used with diesel fuel containing more than 15 ppm sulfur.

Cummins reacted to the introduction of API CK-4 and API FA-4 by issuing its CES 20086 list of API CK-4 registered oils[14] and CES 20087 list of API FA-4 registered oils.[15] Valvoline oils are preferred.

Ford does not recommend API CK-4 or FA-4 oils in its diesel engines.