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Regulators have been tightening limits on diesel emissions; however, those rules do not mention oil at all. (igoriss/Getty Images)

Engine manufacturers, under pressure from the Environmental Protection Agency and California Air Resources Board, are redesigning hardware and duty cycles to meet ever-tighter emissions targets, especially at low load and idle. To keep aftertreatment systems from plugging, poisoning or drifting out of compliance, they need oil that can do more with less.

PC-12, the next heavy-duty engine oil category that will start showing up in bulk tanks and invoices in 2027, is touted as a regulatory-driven, technology-aligned upgrade while remaining backward compatible and manageable for fleets.

Regulators have been tightening limits on diesel emissions; however, those rules do not mention oil at all.

“The EPA doesn’t regulate oil. They regulate emissions,” said Karin Haumann, OEM technical manager with Shell Lubricants. “Engine manufacturers must meet those rules with new hardware and duty cycles and then they come to us and say, ‘We need an oil that can support this.’ ”

PC-12 is an abbreviation for Proposed Category 12, representing the 12th oil category developed by the American Petroleum Institute. It is set to be the first heavy-duty engine oil category introduced since the CK-4 and FA-4 categories in 2017.

PC-12 is set to be released in two service categories. One will be FB-4 for lower viscosity oils, such as 10W-30 and below. It will be compatible with newer engines designed to use FA-4 engine oils. The second will be CL-4, the successor to CK-4, and it will be compatible with older engines, which will help smaller fleets and owner-operators. CL-4 will be used for higher viscosity oils, such as 15W-40 or 10W-30.

OEM Request

Through the Engine Manufacturers Association, OEMs set out what they needed. During a session panel on the new proposed category at American Trucking Associations’ Technology & Maintenance Council Annual Meeting and Transportation Technology Exhibition in Nashville in March, Matt Gieselman, director of engine oils and fuels technology for Lubrizol, and Waleed Salama, senior service engineer at Detroit Diesel, described the request in terms of three broad goals.

The first was better protection for aftertreatment from what Gieselman called SAPS, an acronym describing the class of chemicals such as sulfated ash, phosphorus and sulfur, along with metals such as calcium and zinc.

“These elements will mess with and foul up the function of aftertreatment devices,” he said. “Most notably, [they] will tend to clog, over a long period of time, diesel particulate filters.”

Gieselman explained these ingredients are very effective at neutralizing acids, preventing wear and controlling oxidation; however, over time, they contribute ash that accumulates in diesel particulate filters and elements that can coat or poison catalysts in diesel oxidation catalysts, SCR units and ammonia slip catalysts. OEMs asked for a tighter “chemical box” that would reduce these harmful components in the oil. In PC-12, for example, the maximum allowed sulfated ash content is scheduled to fall from 1% to 0.9%, while phosphorus and sulfur limits also move downward.

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(Detroit Diesel Corp. via broadhead)

The second goal was better survival under heat and time. As engines run hotter and fleets push for extended oil drains, the oil must resist oxidation and thickening while keeping deposits under control. Gieselman noted the PC-12 engine tests showing more oxidation control illustrate how much more is being asked of the oil.

The third goal was the ability to use thinner oils, where appropriate, to improve fuel economy. Comparative data shows that moving from 15W-40 CK-4 to 10W-30 in the same category can yield from 0.5% to 1.5% fuel economy improvement.

“With our testing, we’ve seen expectation of approximately 3% fuel efficiency improvement at the engine level, which equals significant savings,” Salama said.

OEMs were clear, however, that fuel efficient oils could not be granted easier standards. CL-4 and FB-4 products must both pass the same severity of wear, oxidation, deposit and aeration tests.

“The request for performance never goes down,” said Shawn Whitacre, Chevron Lubricants’ principal engineer. “There’s never ‘you’re using less SAPS so you’re allowed to deliver less performance.’ It’s always the other way around … more performance and less SAPS.”

What’s Changed

Field evidence from Lubrizol and others suggests that prototype CL-4 and FB-4 oils are meeting the performance and protection challenges. In fleet trials on modern trucks and tractors, oil analysis trends show lower oxidation numbers at equivalent mileage compared to CK-4 baselines, while iron wear rates remain similar. Gieselman noted that CL-4 and FB-4 formulations held oxidation in check more effectively than a CK-4 reference, while wear metal levels stayed within the same scatter band.

PC-12 is being built as a long-term oil category, not just a quick response to the latest emissions rule, Haumann noted, adding that industry chose its test hardware with durability in mind so the specification can stay in place for many years without a complete overhaul.

“We intentionally evaluated all the hardware that we need for the category to last [until] 2040,” she said, “and we’ve intended for this category to last until 2040. … We’re in a holding pattern where we can maintain until 2040 unless the engine manufacturers say what we have now is not good enough.”

Backward Compatibility

For fleets and owner-operators, it’s about how these changes affect the shop floor. Many operations have a mix of ages, engine makes and duty cycles. They also remember how the introduction of PC-11 in 2016 created a new kind of complexity.

“When PC-11 came into the market, it went from a single category, CJ-4, to a split category, and the category was split on viscosity,” said Darryl Purificati, senior technical adviser, OEM and automotive, at Petro-Canada Lubricants. “There were conventional viscosity grades, and there were low HTHS, or high temperature, high shear, oils. Those oils were designed specifically for fuel economy in mind.”

The trouble, he recalled, was not at the refinery or the lab but at the shelf. A 10W-30 could now be either CK-4 or FA-4. Shop personnel who were used to grabbing “any 10W-30” now had to be sure they had the right category for a given engine.

“That complexity was a step change for the industry that required a large focus on the education piece,” Purificati said. “Previously, they just required a 10W-30, they would go to the shelf, grab a 10W-30 and perform their maintenance. Now they had to understand which 10W-30 was required for which unit.”

From his perspective, PC-12 continues that split but does not raise the bar again in terms of confusion. The work fleets did over the last decade to understand CK-4 versus FA-4 is still valid.

“The complexity is mitigated by the backward compatibility,” he said. “Moving from CK-4 to CL-4, the CL-4 oils are backward compatible with all previous categories. In other words, if you purchase and use a CL-4 in your unit, it covers the CK-4 specs and regulations, and it covers the CJ-4 and the CH-4 back through time.”

FB-4, like FA-4, is a different case. Because these oils sit in a lower HTHS band, they are not automatically suitable for older engines. Engine manufacturers are validating them for newer designs, and their owner’s manuals and service bulletins will continue to specify where they are allowed.

Detroit Diesel and Cummins both showed service strategy charts that map their engine families to the C and F categories. For newer platforms, especially those introduced after 2027, FB-4 and other low-HTHS oils are likely to appear more often. For older units, CL-4 will be the safe choice.

Fleet Advantage

That combination of tighter chemistry, higher performance demands and backward compatibility could still feel like a disruption; however, Haumann and Purificati both argue that fleets can convert it into a planning opportunity if they start early and lean on the right partners.

The first step is to inventory assets and priorities. Maintenance directors need to know not only what engines they have, but what they are trying to optimize. Some fleets put a premium on extended drains. Others are more focused on fuel economy or on keeping the lubricant program as simple as possible. Those priorities will shape decisions about whether to run a single CL-4 product across the board or to introduce FB-4 in specific engines to gain extra efficiency.

“I would recommend looking at the asset makeup, the age of your engines, the makes, the age, and then define your priorities. ‘Do you want the longest extended oil drain intervals?’ ‘Do you want the best fuel economy?’ ” Haumann advised. “Talk with your oil marketer … so that when the new products come out, you know what you’re ordering is most suitable for you to meet your goals.”

Purificati concurred.

“They are the people that know best how to transition to a new fluid and how to manage that process,” Purificati said. “It is not the first category change we have been through, and it is imperative for people to lean on the expertise of the people. … [They are] the one-stop shop that will help from start to finish through the transition phase.”

Staff Reporter Keiron Greenhalgh contributed to this article.