In April 2017, Ford updated their position on CK-4 oils for diesel engines to the following:
Due to Ford Motor Company’s commitment to continuous improvement, Ford will keep our customers informed of changes to this situation. On Dec. 1, 2016 the American Petroleum Institute (API) began licensing two new diesel motor categories: CK-4 and FA-4. API CK-4 was released to replace CJ-4. FA-4 is a low-viscosity diesel oil released for diesel engines designed for such oils.
Due to its low viscosity, FA-4 should not be used in any Ford diesel vehicles at this time.
Ford will not be recommending the use of CK-4 motor oils in any Ford diesel engines, new or old. Ford testing has shown some CK-4-type formulations have shown inadequate wear protection compared to CJ-4 formulations developed and licensed before 2016.
Ford now recommends using oils that meet our OEM specification, Ford Material Engineering Specification WSS-M2C171-F1, similar to many other diesel engine manufacturers who also only recommend oils meeting their own OEM specifications. WSS-M2C171-F1 contains all the requirements of CK-4 with additional Ford requirements for added engine protection. The customer should use an oil that meets this Ford specification. For a short period of time, an oil showing CJ-4 in the API donut without showing CK-4 would be acceptable for service even if it’s not showing WSS-M2C171-F1. This oil would most likely be an older CJ-4 formulation, developed and licensed prior to 2016.
Years of experience and Ford testing has shown that these older CJ-4 formulations provide the necessary engine protection in all Ford diesel engines where CJ-4 and older categories are recommended. Some of these CJ-4 oils could be available for up to a year after Dec. 1, 2016. To ensure you protect your Ford diesel engine, use Motorcraft Super Duty Motor Oil meeting API CJ-4. Check this Motorcraft website for diesel motor oils approved by Ford Motor Company for servicing Ford diesel engines:
The customer should not use oils labeled CK-4/SN unless they have been approved by Ford Motor Company to meet WSS-M2C171-F1. This is possible with SAE 10W-40, 15W-40, 5W-40 and 0W-40 oils. At this time, SAE 10W-30 oils showing CK-4/SN cannot meet WSS-M2C171-F1 and should be avoided. So, if unsure, avoid CK-4 oils that also shows SN in the API donut.
NEVER use C-category oils in gasoline engines with emissions catalysts, even if they show SN in the API donut (like CK-4/SN, CJ-4/SN, etc.), UNLESS they also show the Starburst symbol on the front label stating, “American Petroleum Institute Certified for Gasoline Engines.” SN oils without a Starburst symbol can be harmful to your vehicle’s catalyst and reduce fuel economy.
Courtesy of Ford.