Diesel Engine Oil for Semi Trucks: CK-4, FA-4, Drain Intervals and Oil Analysis

The direct answer: use the oil category and viscosity the engine maker specifies for that engine, change it on the interval the maker publishes for the way the truck actually works, and use oil analysis if you want evidence instead of a guess. API CK-4 is the current category that also covers older engines. API FA-4 is a separate, thinner category for certain newer engines and, in the American Petroleum Institute’s words, is “not interchangeable or backward compatible” with CK-4 and earlier oils.

CK-4 and FA-4: what the labels mean

Both definitions below are from the American Petroleum Institute’s oil category list.

• API CK-4 describes oils for high-speed four-stroke diesel engines designed to meet 2017 model year on-highway emission standards “as well as for previous model year diesel engines.” API states that CK-4 oils exceed the performance criteria of CJ-4, CI-4 and CH-4 and can lubricate engines calling for those categories. It is the oil most trucks on the road use.
• API FA-4 identifies certain XW-30 oils formulated for select engines designed to meet 2017 model year greenhouse gas standards. API’s instruction is to “refer to engine manufacturer recommendations to determine if API FA-4 oils are suitable for use.”

In practice: CK-4 is the safe default wherever the engine maker allows it. FA-4 goes only into an engine whose maker approves it. Putting FA-4 into an engine that was not designed for it is the mistake to avoid, particularly in a mixed fleet filling from one bulk tank.

Viscosity is a separate choice

The category (CK-4, FA-4) says what tests the oil passed. The grade (15W-40, 10W-30, 5W-40) says how thick it is cold and hot. Many engine makers approve more than one grade and choose by climate and fuel-economy goals. Use a grade the engine’s service literature lists. Engine makers also publish their own oil specifications on top of the API category; the bottle or the product data sheet lists which it meets.

What actually sets a drain interval

The engine maker’s published interval for the engine model, adjusted for duty cycle. The same engine has different intervals for long-haul, regional and severe service, because the things that wear oil out are different in each:

• Idle time and short trips load the oil with soot and fuel without adding many miles. A truck that idles heavily needs its oil judged by engine hours or fuel burned, not the odometer.
• Heavy loads, grades and heat oxidize oil faster.
• Dust — construction and agricultural work — brings silicon past a tired air filter.
• Fuel dilution from injector faults or frequent regeneration thins the oil.
• Fuel sulfur. API notes that using CK-4 with fuel above 15 ppm sulfur may affect aftertreatment durability and the drain interval.

Our maintenance schedule guide covers building intervals around the work a truck does.

What an oil analysis report shows

A lab report on a small sample taken at the drain. The value is in the trend across several samples from the same engine; a single report is a snapshot.

Reading	What a rise usually points to
Iron	Cylinder liners, gears, valve train wear
Copper, lead, tin	Bearings and bushings. Copper alone can also come from an oil cooler core.
Aluminium	Pistons, some bearings and housings
Chromium	Piston rings
Silicon	Dirt getting in through the intake. Check the air filter and its piping.
Sodium and potassium	Coolant entering the oil: EGR cooler, oil cooler, head gasket or liner seals
Fuel dilution	Injectors, or fuel washing past the rings
Soot	Combustion quality, EGR and air-filter condition, an over-long interval
Viscosity out of grade	Thinner: fuel dilution or shear. Thicker: soot or oxidation.
Base number falling	The oil’s reserve against acids is running out; the interval is at its limit

The coolant markers matter most. Coolant in the oil destroys bearings, and sodium or potassium shows in a lab report well before the oil turns milky or the coolant level becomes obvious. See coolant loss with no visible leak.

Getting a sample that means something

• Sample with the oil warm and recently circulated.
• Take it the same way every time — mid-drain, or from a sampling valve — not the first oil out of the plug.
• Use a clean bottle and label it with engine hours, miles on the oil and any oil added.
• Sample at each drain so the lab has a trend to compare.

Extending drains

Longer intervals are possible on some engines in some duty cycles, and the engine maker’s literature says which. The evidence that an extension is working for a particular truck is a series of clean oil analysis reports, not a claim on an oil bottle. Extending without sampling is a decision made blind.

What we do at an oil service

Oil and filters to the engine maker’s specification, with the rest of the fluids and a look at what the truck is telling us while it is there. Tell us if you want a sample pulled for analysis. See oil change and fluid service and preventive maintenance, or call (916) 571-8888.

Common questions

Can I use FA-4 oil in an older truck?

Not unless the engine maker approves it for that engine. API states FA-4 oils are not interchangeable or backward compatible with CK-4, CJ-4, CI-4 PLUS, CI-4 and CH-4 oils.

Can I use CK-4 where CJ-4 was specified?

API states that CK-4 oils exceed the performance criteria of CJ-4 and can lubricate engines calling for it. Check the engine maker’s own specification as well.

Is 15W-40 or 10W-30 better?

Neither in general. Use a grade the engine maker lists for the engine and the temperatures it works in.

Is oil analysis worth it for one truck?

It costs little compared with an oil change and can show coolant, fuel or dirt in the oil before there is any other sign. For an owner-operator whose income is one engine, that early warning is the point.

Category definitions quoted from the American Petroleum Institute. General guidance only; the engine manufacturer’s service literature governs oil selection and intervals.