Short answer
API FA-4 covers specific SAE xW-30 oils for select high-speed four-stroke diesel engines designed to meet 2017 on-highway greenhouse gas standards, with first licensing on December 1, 2016. FA-4 oils are blended to an HTHS viscosity between 2.9 and 3.2 cP to reduce friction. API states they are not interchangeable or backward compatible with CK-4, CJ-4, CI-4 PLUS, CI-4 or CH-4, so use them only where the engine maker recommends FA-4.
Why a lower-viscosity diesel category
U.S. greenhouse gas rules for heavy trucks pushed engine makers to cut fuel consumption, and lower oil viscosity is one way to reduce internal friction. Some engine makers designed 2017-and-newer on-highway engines to run on thinner oil and asked for a category that would define it.
FA-4 came out of the same development program as CK-4, so the two share the newer oxidation and aeration tests. The difference is the HTHS window: FA-4 sits between 2.9 and 3.2 cP, where CK-4 must be at least 3.5 cP.
Fuel and application limits
API describes FA-4 as designed for on-highway diesel fuel up to 15 ppm sulfur and does not recommend it for fuels above that level. That, plus the viscosity range, is why FA-4 is generally an on-highway oil and is not used in older or off-road engines unless the maker specifically approves it.
Engine makers took different positions
Not every manufacturer adopted FA-4. Some published FA-4-based specifications for particular engines, while others limited their recommendations to CK-4 or approved FA-4 only for certain engine families. The only reliable guide for a given engine is that manufacturer’s current recommendation.
Mixed fleets
Because FA-4 cannot go into engines that call for CK-4 or earlier categories, fleets running a mix of model years often stock a single CK-4 oil to avoid misapplication, or segregate and clearly label FA-4 where they use it.
Frequently asked questions
Can FA-4 go in my 2010 diesel truck?
API says FA-4 is not backward compatible with CJ-4 or earlier categories, so it should not be used in an engine that calls for those unless the engine maker says otherwise.
Does FA-4 come in 15W-40?
No. FA-4 is limited to xW-30 grades such as 5W-30 and 10W-30 blended within the 2.9 to 3.2 cP HTHS window.
General maintenance information. Always follow your vehicle or equipment manufacturer’s requirements and the current product data. Questions? Email [email protected].