This is the first I've heard about this, but definitely keep it in mind in the future.
CK-4 Diesel Update
Recently diesel engine oils changed from the decade old CJ-4 formulation to the new CK-4 formulation. While intended to be an upgrade, the new formulation has been met with resistance and even an outright “do not use” warning from Ford Motor Co.
The new CK-4 specification went into effect in December of 2016. Just before the launch of this new specification, Ford issued a service bulletin stating that Ford 6.7L diesel engines should not use the new CK-4 oil. In fact, Ford issued a new oil specification closely aligned with the previous CJ-4 spec and called for the use of CJ-4 oils meeting the new Ford specification for their 6.7L diesel engines. Ford identified wear issues as the reason for abandoning the new CK-4 oil specification.
What is the underlying issue with the CK-4 oil? Previous API diesel oil specifications, like the decade old CJ-4, had to pass the ASTM Sequence IIIG valve train wear and oxidation test. This is no longer true of the new CK-4 specification. The Sequence IIIG test measures flat tappet cam and lifter wear as well as other parameters. The significance to both Ford and other users of diesel engine oils is directly tied to the absence of the Sequence IIIG test.
Diesel engine oils have historically contained higher levels of the anti-wear additive ZDDP. With more oil marketers wishing to supply “universal” oils that meet both API SN requirements for gasoline engines as well as API CK-4 requirements for diesel engines, the levels of ZDDP have dropped in some CK-4 oils. Without the Sequence IIIG flat tappet cam wear test, it appears some of these lower ZDDP diesel oils are causing increased wear in diesel engines. This should be a word of caution for all users of diesel engine oil, regardless if you’re using it in a 6.7L Ford or not.
Engine Pro Technical Committee with special thanks to Driven Racing Oil