Article originally appeared in FleetOwner. Reprinted with permission.

The transition to CL-4 oils is expected to be backward-compatible, but FB-4 use may depend on OEM approvals for specific engines and applications.

Tanks and bottles of heavy-duty diesel engine oil throughout North America will soon look a little different.

The American Petroleum Institute (API) plans to officially roll out its next heavy-duty lubricant performance categories, API CL-4 and its fuel-efficient variant API FB-4, in January 2027. The new categories set new limits for engine oil performance to best meet the needs of modern engine hardware and will effectively replace the existing API CK-4 and API FA-4.

For fleets, the transition should be relatively seamless. However, it’s worth staying informed about the change and what it means for lubricant formulations and the impact on your vehicles, and proactive preparation and education can help you seize the full range of benefits. In this article, we’ll explore the performance requirements of API CL-4 and API FB-4 and how to select the best available formulation to suit your business needs.

API CL-4 engine oil basics and emissions-driven performance changes

API CL-4 and FB-4 have been developed in response to new regulatory standards. Development began coinciding with the 2022 announcement of new emissions standards set to take effect in 2027 for new diesel engines. Goals included a 75% reduction in nitrous oxide (NOx) emissions, a 50% reduction in particulate matter, and extended service life requirements for aftertreatment vehicles. While exact requirements for these reductions may fluctuate slightly, the formulation priorities of API CL-4 fluids remain designed to contribute to progress on these fronts.

Overall, the category will deliver elevated performance in terms of piston deposits, oil consumption, valvetrain wear, shear stability, oxidation control, elastomer compatibility, adhesive wear, and aftertreatment system protection. Simultaneously, the category aims to deliver new value to the marketplace in terms of enhanced engine protection, longer oil drain intervals, and enhanced fuel economy for end users.

Lower-viscosity oils and fuel economy gains for fleets

That last point is important: The new performance category represents an opportunity for fleets to attain greater fleet-wide fuel economy by selecting optimized formulations.

API CL-4 will arrive along with API FB-4, a split performance category that classifies lower-viscosity lubricants that can contribute to enhanced fuel economy gains. While API CL-4 fluids are backward-compatible for all trucks on the road, API FB-4 fluids will be designed specifically for new-model engines and maintain all the same performance characteristics of API CL-4 options. It’s expected that some original equipment manufacturers (OEMs) will allow for the use of API FB-4 fluids for regular service, though some will not.

This is where early conversations with your oil supplier can be beneficial. Ideally, they should be able to help you identify if API FB-4 fluids are a better choice for your fleet, or at least portions of it. And it’s not just API FB-4 formulations that can deliver enhanced fuel economy; making a switch within the API CL-4 category can be significant as well. Transitioning from 15W-40 fluids to 5W/10W-30 formulations can lead to real gains, and throughout a lifetime of service, it can add up to notable operational cost savings.

Oil drain analysis tips for tracking wear, contaminants, and uptime

No matter the viscosity grade you choose, API CL-4 lubricants will be fundamentally different than their API CK-4 counterparts.

This is an important point for fleets that perform oil drain analyses. By sampling and analyzing used oil following an oil change, fleets can monitor unwanted contaminants or metal content in their oil drains, which can indicate other issues. Many OEMs issue condemnation limits—for example, a certain parts per million of iron in an oil sample—to help fleets catch any issues before they become larger problems.

But API CL-4 will have a different baseline profile than API CK-4 fluids, and it may, in some instances, contribute to new variations in the signature of your oil samples. This is another reason to start early conversations with your fluid provider, as they should be able to help you better understand what to look for.

With the imminent arrival of API CL-4 and API FB-4, the future of heavy-duty diesel engine oils is almost here. Forward-thinking fleets shouldn’t wait until December to begin having conversations with their fluid suppliers. By selecting optimized lubricants best suited for the makeup of your fleet, you’ll gain a significant opportunity to lower the total cost of ownership for the trucks on which your business is built.

The impact of API CL-4 engine oil on fleet maintenance, fuel economy, and uptime