Nearly a decade in the making, the new specifications were necessary to enable diesel engine manufacturers to meet strict emissions and fuel-mileage requirements and to address the diverse needs of engine technologies as they relate to older, newer, and in-use applications both on and off the highway.
In order to help you distinguish between the two, API developed two distinctly different Service Symbol Donuts. The CK-4 Donut will look like the current CJ-4 API Donut, while the FA-4 Donut will feature a shaded section to help differentiate it from the new CK-4 oils.
If you're driving an older semi, it's important to note that only the CK-4 category of oil will be compatible with your truck's engine. The FA-4 is intended specifically for use in newer on-highway diesels. However, they'll improve both emissions and fuel mileage in newer engines.
That said, the CK-4 category will replace the current CJ-4 API oils, and will provide a number of benefits over the CJ-4 oils, including improved shear stability, increased resistance to oxidation, and better aeration control.
If you're unsure which category you should use, check your owner's manual and/or contact your engine manufacturer. You can also check out the API website for more detailed information on these two new service categories. If you still have questions, check out Answers to 3 Key Questions on the 2 New Oil Categories by HDT Trucking Info editor, David Cullen.
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