http://rvtravel.com/rv-travel-newsletter-issue-748/

Quote: Get ready for changes in engine oil specifications, coming at you December 1.
The old standard many are used to, CJ-4, will give way to a new CK-4 standard.
CK-4 is said to withstand engine temperature breakdown better than its predecessor, and is fully “backwards compatible,” meaning you can use it in rigs calling for the CJ-4 standard.
Another new diesel oil, FA-4, will be available for NEWER rigs, offering greater fuel economy. For the most part, these are NOT backwards compatible.
All this is courtesy of government fuel efficiency standard tinkering. :Unquote

Very Inreresting indeed!