CK-4 vs FA-4 diesel oil: the differences that matter
API introduced CK-4 and FA-4 in December 2016 to replace CJ-4. They look similar on the shelf, but they are not interchangeable. Using the wrong one can be a warranty problem and, in older engines, a wear problem.
The short version
| API CK-4 | API FA-4 | |
|---|---|---|
| Backward compatible | Yes, with CJ-4, CI-4 Plus and older | No, only for engines designed for it |
| Typical grades | 15W-40, 10W-30, 5W-40 | 10W-30, 5W-30 (lower HTHS viscosity) |
| HTHS viscosity | 3.5 cP or higher | 2.9-3.2 cP |
| Aimed at | Almost all on- and off-road diesels | Some 2017+ heavy-duty highway engines, for fuel economy |
| Pickups | The usual choice | Generally not specified for diesel pickups; check your manual |
What HTHS means and why it matters
High-temperature high-shear (HTHS) viscosity is how thick the oil stays in the hottest, highest-load parts of the engine, such as bearings and the cam. FA-4 is deliberately thinner there to cut friction and improve fuel economy. Engines designed for it have the clearances and parts to cope. Older engines, and most pickup engines, were designed around the thicker oil film of CK-4.
Which one does my pickup use?
Always follow the owner's manual. As a general pattern:
- Ford 6.7 Powerstroke: Ford specifies its own approvals (such as WSS-M2C171-F1 for 10W-30/15W-40 on recent years); CK-4 oils that carry that approval.
- Ram 6.7 Cummins: Cummins CES 20086 (CK-4 level), typically 15W-40 or 5W-40 in cold climates.
- GM Duramax: dexos D approved oil, typically 15W-40 or 5W-40 per climate.
Look for both the API donut and the manufacturer approval on the bottle.
Low ash and your DPF
Every oil leaves a little ash when it burns. Ash cannot be burned off in a DPF regen, so it builds up for the life of the filter. CK-4 limits sulfated ash to 1.0%, part of why modern oils are kinder to DPFs than older specs. Oil burning from worn rings, valve seals or a leaking turbo seal adds ash fast. See DPF regeneration.
Change intervals
Use the truck's oil life monitor and the severe service schedule if you tow heavy, idle a lot, or drive short trips. Diesel oil gets loaded with soot quicker than gasoline engine oil. An oil analysis every few changes is cheap insurance and shows fuel dilution, coolant and wear metals early.
Related trouble codes
FAQ
Can I use FA-4 instead of CK-4?
Only if your engine manufacturer specifically allows it. FA-4 is not backward compatible and is generally not recommended for diesel pickups.
Can I use CK-4 in an older diesel?
Yes. CK-4 is backward compatible with engines that called for CJ-4, CI-4 Plus and older categories.
Is 15W-40 or 5W-40 better?
It depends on climate and your manual. 5W-40 flows better in cold starts, while 15W-40 is the traditional choice in warmer weather. Many manuals allow both.
Why does diesel oil turn black so quickly?
Diesel combustion makes soot, and the oil's detergents hold it in suspension. Dark oil soon after a change is normal and shows the detergents are working.
More diesel guides
General guidance only. Check the service information, recalls and bulletins for your exact vehicle. Costs are rough 2026 US estimates, not quotes. Need a number for your truck? Try the repair cost estimator.