If you want to follow Ford's advice, don't use Rotella T6 on the shelf after October 2016 (all T6 in the new bottles is CK4, even though it says CJ4 on bottle), as Ford thinks it has too little P for adequate wear protection.
If you want to use a 5w40 that Ford thinks is ok, easiest acceptable ones to get are either Mobil 1 TDT 5w40 or Mystik 5w40. These two 5w40 HDEOs are still CJ4 (and, importantly, aren't changing to CK4), and have more than 1000 ppm phosphorous (P) and are OK per Ford.
Ford has testing ongoing and hopefully will approve more 5w40s this Spring. Many more 10w30 and 15w40 oils are already on their approved list.
Ford Motor Company CK-4/FA-4 Ford Position Statement
Starting on December 1, 2016 the American Petroleum Institute (API) will begin licensing
two new diesel motor categories CK-4 and FA-4. API CK-4 is being released to replace
CJ-4. FA-4 is a low viscosity diesel oil released for diesel engines designed for a lower
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. Testing Ford has done on some CK-4 formulations have shown inadequate
wear protection compared to CJ-4 formulations developed and licensed before 2016.
Like many other diesel engine manufacturers, with their own internal OEM specification,
Ford will now be recommending oils that meet an OEM specification, Ford Material
Engineering Specification WSS-M2C171-F1. The customer should use an oil showing
that it meets this specification.
An oil showing CJ-4 in the API donut without showing CK-4 would be acceptable for
service even if not showing WSS-M2C171-F1. This oil would most likely be an older CJ-
4 formulation, developed and licensed prior to 2016. These oils could be around for
about a year after CK-4 licensing begins, December 1, 2016. Field experience and Ford
testing has shown that these older CJ-4 formulations provide acceptable 6.7L engine
To insure you protect your Ford diesel engine use an oil meeting Ford specification
WSS-M2C171-F1 like Motorcraft Super Duty Motor oil.