The oil that flows through diesel engine galleries has little in common with the fluids of days gone by, largely because of evolving emissions standards.

When exhaust gas recirculation (EGR) systems were rushed to market in 2002, CI-4 oil had to contend with lubricant-thickening soot; an interim CI-4 Plus category was introduced when those soot loads proved to be higher than expected. And the CJ-4 oils that followed them placed a 1% cap on sulfated ash that would otherwise clog diesel particulate filters.

Work is now underway to develop an oil category that will help engine manufacturers meet the tightening NOx limits applied to Model Year 2027 equipment and beyond.

Just like the CK-4 and FA-4 oil available in bottles and barrels today, you can expect the category to split in two – with “backwards compatible” formulas for legacy equipment, and a separate option to support the newest engine technology. It’s officially known as Proposed Category 12 for now, but the API is widely expected to dub the oils CL-4 and FB-4 when they hit store shelves on Jan. 1, 2027.

Two paths for oils

Getting the trucking industry to recognize the need for two distinct oils was no small matter, of course.

“There was a lot of discussion – a lot of very robust discussion – on how we do that without creating mass confusion,” says Karin Haumann, Shell’s OEM technical services manager, who also serves as chairwoman of API’s new category development team. “There was the challenge of creating a lower-viscosity oil with the same wear protection.”

Essentially, CK-4 and FA-4 oils must pass common performance tests and limits, but the thinner FA-4 is needed to help manufacturers meet tighter limits on greenhouse gas emissions. And since the formulas need to address unique hardware needs, they can’t be interchanged.

“The two categories – CK-4 and FA-4 – are split by viscosity grades, and there’s a bit of a gap in between the High Temperature High Shear windows,” she explains. “That was primarily so that the OEMs felt comfortable that each oil was kind of going to be used in the correct place.”

In other words, the oil pressure warning light might shine if an FA-4 oil is added to an older engine that requires a 15W-40 CK-4.

A diverging oil category

The differences in today’s split category were only the beginning. Proposed Category 12 will likely “diverge” even further.

“Oils will look more different now than I think they did in the transition from CJ-4 to CK-4,” she says. “We’re focusing really heavily on backward compatibility for … the CL-4 portion, but with the FB-4 we’re kind of diverging away from that backward compatibility.

“We’re going to kind of let go of some of those legacy things that aren’t necessary for the new hardware, like high soot control.” Expect soot-handling requirements to be more like those associated with CI-4 and CH-4 oil categories.

Introducing FB-4 oils with OW-20 and 5W-20 weights will help to “future-proof” the category for engine manufactures, too.

Oil temperatures and emissions

Steps to control oxidation will be particularly important for manufacturers that want to increase oil temperatures in the name of lowering emissions, she adds.

“Oil viscosity is a function of temperature,” Haumann explains. “One way that they can lower the viscosity is to increase temperature, which of course exacerbates oxidation. The more heat tolerance we’re able to build into the oil, the more freedom [manufacturers] have to do those types of things on the design side to help with their emissions targets.”

Manufacturers that don’t take advantage of the increasing temperatures, meanwhile, might instead be able to offer longer oil drain intervals in certain applications. (“That’s kind of for the OEM to decide.”)

But the performance limits on the oil will also play a role in supporting the U.S. Environmental Protection Agency’s calls for emissions system warranties that last 2.8-4.5 times longer than those in place today.

“Now the OEMs are really focused on, ‘Let’s look at what kind of chemistries you’re using in the oil, and what those limits are — to make sure they’re able to extend the useful life of those aftertreatment systems,’” Haumann says.

New tests for oils

There will even be changes in tests the new oils must pass.

A new Ford valvetrain wear test is just one example of that. And to compound matters, it comes just as those who develop the oils need to lower ZDDP levels, likely reducing the use of phosphorous that happens to be an anti-wear compound.

“So we’re adding a wear test that doesn’t exist now, and we’re also limiting the amount of one of the anti-wear components that we’re able to utilize,” Haumann says. “Trying to do both of those at the same time, it’s going to require new chemistries.”

A Detroit Diesel scuffing test, which wasn’t quite ready when CK-4 and FA-4 were being introduced, will also be added to the list. Granted, several of today’s oils already meet that manufacturer-specific requirement.

But two traditional tests are being phased out because the related engine parts are too hard to come by — the Mack T11 test that measures soot handling in stop-and-go applications, and the Mack T12 test that helps measure ring liner wear. The choice here will be to develop new tests to replace them or see if the results are redundant. Maybe the required information can be collected from the results available through other tests.

“We’ve got some tests that measure overlapping things,” Haumann explains.

When it comes to the T11 test, the work will likely involve redesigning test cycles for other engine platforms such as a Cummins ISM or ISB when looking at soot-induced viscosity increases.

Why we need a category

This is all about setting a minimum standard for the fluids that you buy. With the industry specification in place, fleets and owner-operators can shop for oils based on factors like the equipment being used, recommended viscosity grades, and environmental conditions.

“You don’t have to worry about things like shear stability or soot control or those things because we have those minimum standards,” Haumann says. But the shear stability helps retain the oil grades found on the label. And the steps to resist oxidation offset rising engine temperatures that would otherwise thicken the oil.

There will be months of work before we reach that final step, though.

While performance requirements have been defined, the team now needs to decide on the reference oils that will be used in the tests, and ultimately reconcile any wish list against available funding. Even when the work is done, API has a mandatory year-long waiting period to ensure that all oil manufacturers and marketers have the time to access the same tests. Bigger industry players can’t simply use their added resources to rush new oils to market ahead of competitors.

The new jugs and barrels of oil will be available at parts counters by Jan. 1, 2027. Engine manufacturers will have the supplies of FB-4 earlier than that, to support the factory fills on the first Model Year 2027 engines to roll off the assembly line.

“We just want to be able to make sure that we’re looking forward enough in the future that we don’t have to do this any more frequently than it’s required,” Haumann says of all the steps that will come before then. The final category is meant to last close to a decade.

“It’s a big process.”