After several years of development, we're nearing the finish line for the next heavy-duty diesel engine oil specification.

The proposed PC-12 category, which will succeed today’s CK-4 and FA-4 oils, has cleared its final technical hurdles and is moving toward first licensing on January 1, 2027.

“PC-12 right now is really on the final step,” said Bill O’Ryan, director of the Engine Oil Licensing and Certification System at the American Petroleum Institute. “All of the ballots through ASTM and the API Lubricants Group have passed.”

O'Ryan was speaking during a panel discussion at the American Trucking Associations’ Technology & Maintenance Council Annual Meeting in Nashville.

Those approvals clear the way for the two service categories that will make up PC-12: API CL-4, which will replace CK-4 as the primary backward-compatible oil, and API FB-4, designed for newer engines that can take advantage of lower-viscosity oils for fuel-economy gains.

Before oils can appear in the market, the industry must go through a mandatory waiting period that allows oil marketers and additive suppliers time to finalize formulations and prepare products for licensing.

And even though licensing is expected to open on January 1, 2027, don't expect to see a flood of the new oils in the market immediately.

Why a New Oil Category Is Needed

New diesel engine oil categories typically arrive every decade or so as engines evolve. PC-12 is no exception.

Truck and engine manufacturers determined that future engines would need improved oil performance. Those needs were submitted to API through the Truck and Engine Manufacturers Association, which requested development of a new category.

From there, the proposal moved through a series of industry committees coordinated by API and ASTM, involving engine manufacturers, additive companies, oil marketers, and testing laboratories.

The development committee then must develop tests and performance limits that ensure oils can meet those requirements.

For PC-12, the main targets were:

• Better oxidation resistance
• Improved wear protection
• Compatibility with modern engine materials and seals.

But achieving those goals isn’t simply a matter of adding more additives.

Developing a new engine oil specification is partly a chemistry exercise and partly a balancing act. Additives that solve one problem can sometimes create another.

“Putting it all together is the challenge,” said Karin Haumann, OEM technical services manager for Shell Global Solutions. “It’s a multivariate problem you’re trying to solve. You’re trying to balance performance in sometimes competing areas.”

Will EPA 2027 Rules Affect PC-12?

The new oil category is expected to arrive around the same time as EPA’s 2027 heavy-duty emissions standards. With the EPA in the process of making some revisions to those regulations, that has raised questions about whether changes could affect PC-12. According to the panelists, the answer is no.

“The development of PC-12 is a technical exercise,” Haumann said. “While some of that might be driven by regulatory targets, it’s not driving the PC-12 process.”

In other words, the category is being developed to meet the needs of modern engines regardless of how emissions rules evolve.

Heather DeBaun, vehicle fluids technical specialist at Traton R&D, said the improvements built into PC-12 will remain relevant for both current and future engines.

“It will offer improvements that we’re looking for in areas of durability, oxidation control, and wear protection,” she said.

Extensive Field Testing

While much of the work behind a new oil category takes place in laboratories and engine test cells, oil companies are also conducting extensive real-world testing.

Haumann said Shell alone has accumulated about 8 million miles of on-road fleet testing with candidate PC-12 oils, along with more than 12,000 hours of testing in off-highway equipment.

Those tests include both new trucks and older equipment. Backward compatibility remains an important part of the specification, and fleets want to know that new oils will perform well in existing engines.

Potential Benefits for Older Engines

Although PC-12 is designed primarily for future engines, some of the improvements could benefit older engines as well.

One example is improved oxidation stability, which helps engine oil resist breakdown at high temperatures. That performance improvement can help maintain oil quality over longer service intervals.

“If a fleet wanted to look at extending oil drain intervals, oxidation is important to that,” Haumann said. “This new category offers better performance in that area.”

Additional wear-protection tests built into the category could also benefit older engines.

Still, fleets should always follow their engine manufacturer’s recommendations when selecting engine oil.

Ultra-Low-Viscosity Oils Coming

Like the current CK-4 and FA-4 categories, PC-12 will again contain two different categories.

CL-4 oils will cover the traditional viscosity grades that fleets are accustomed to today. FB-4 oils will support lower-viscosity formulations, intended to help engine manufacturers and truck owners improve fuel economy.

But fleets shouldn’t expect them to dominate overnight.

“It’s going to be a while before those oils become predominant,” DeBaun said. “The engines will be designed to use them, but it won’t necessarily be required.”

Instead, fleets will likely adopt them gradually as new engine platforms enter the market.

What Fleets Should Do Now

With first licensing still more than a year away, fleets don’t need to make any immediate changes. But maintenance managers can start preparing by reviewing their current oil programs and understanding which oils their equipment requires.

“Look at the products that are currently in your fleet and what the recommendations are for your equipment,” DeBaun said.

Once PC-12 oils enter the market, fleets should watch for API certification marks — often called the “donut” symbol — on product packaging. API also maintains an online directory of licensed engine oils where fleets can verify that a product displaying the certification mark is properly approved.

In addition, the organization runs an ongoing aftermarket audit program, sampling oils from the marketplace to ensure they meet the specifications they claim.