Short answer
The core difference is viscosity and compatibility. API CK-4 oils have an HTHS viscosity of at least 3.5 cP and can be used in engines calling for CJ-4 and earlier categories; API FA-4 oils are xW-30 grades held between 2.9 and 3.2 cP and are meant only for engines whose maker specifies FA-4. When in doubt, the engine manual decides, and FA-4 should never be substituted where CK-4 or an older category is called for.
Side by side
Viscosity: CK-4 has an HTHS floor of 3.5 cP and comes in many grades, including 15W-40. FA-4 is limited to xW-30 grades with HTHS from 2.9 to 3.2 cP.
Fuel: API describes CK-4 as formulated for fuel up to 500 ppm sulfur, with a caution about aftertreatment above 15 ppm. FA-4 is designed for on-highway fuel up to 15 ppm and is not recommended above that.
Compatibility: CK-4 can be used where CJ-4, CI-4 PLUS, CI-4 or CH-4 is called for. FA-4 is not interchangeable with or backward compatible to any of them.
What they share
Both categories came from the same API development program, first licensed December 1, 2016. They share the newer engine tests for oxidation and aeration, so the split is about viscosity and where the oil can be used rather than one being a lesser version of the other.
Choosing for a truck or fleet
Start with each engine’s manual or the engine maker’s current service guidance. Some 2017-and-newer on-highway engines list FA-4 as an option; others list CK-4 only. Off-road, agricultural and older engines generally call for CK-4 or an earlier category.
For mixed fleets, a single CK-4 oil in an approved grade covers the widest range of engines. If FA-4 is used for specific trucks, label storage and fill points so it does not end up in an older engine.
Reading the bottle
The top half of the API donut lists the category. Many diesel oils list more than one, such as CK-4 alongside an API gasoline category, so read the full donut and the back label for engine maker specifications as well.
Frequently asked questions
Can I use CK-4 in an engine that allows FA-4?
Many engine makers that permit FA-4 also permit CK-4 in the same engines, but that is the manufacturer’s call. Check the engine maker’s current recommendation.
Can I use FA-4 in an engine that calls for CK-4?
No. API states FA-4 is not backward compatible with CK-4 or earlier diesel categories.
General maintenance information. Always follow your vehicle or equipment manufacturer’s requirements and the current product data. Questions? Email [email protected].