Engine oils that fall under CK-4 are functionally identical to existing synthetic oils used today and will continue to be available for older diesel engines.

But newer diesels often run hotter due to emissions and milage standards, and that’s where FA-4 comes in. It has a lower viscosity than CK-4 oils and holds up better in high-temperature, high-shear environments in which older oil designs might lead to engine damage. This is good news for modern engines, which can run 10° C hotter than older models, but FA-4 does not perform well in older diesel engines and may even cause some models to seize up as the oil goes out of grade.

The new standard has been in development for nearly five years as a collaboration between lubricant manufacturers, such as Shell Rotella, and engine makers, including Cummins and Detroit Diesel. Engine oils bearing the CK-4 and FA-4 labels will be on shelves in December.

“The last time there was a new category was CJ-4, in 2006,” says Dan Arcy, OEM technical manager with Shell Rotella. “There have been a lot of changes in diesel-engine design since.” While there may be some confusion at the auto parts store when picking engine lube, Arcy says that CJ-4 eventually will be phased out in favor of the new standard.