2027 will be here before you know it. And as the calendar changes to the new year, the engine oils you purchase to protect your assets will be changing with it.

The American Petroleum Institute (API) is expected to officially license its next performance category, API CL-4 and API FB-4, in early 2027. Next-generation lubricants meeting the new category’s performance requirements will offer several benefits for trucks on the road, including lowered emissions, elongated diesel particulate filter service lives, and better fuel economy performance.

For forward-thinking fleets, it’s worth preparing in advance for this coming shift in order to seize the opportunities that may be presented by next-generation lubricants. Choosing the right oil type for your specific fleet makeup can deliver significant benefits in terms of fuel savings, the longevity of your trucks, and total cost of ownership. In this article, we’ll break down the nuances of the new category, including available viscosity grades, the critical differences between API CL-4 and API FB-4, and how you can make the right choices.

The importance of the right oil for the application 

API develops new performance categories primarily in order to support the evolving engine hardware you depend on to do business. Because when heavy-duty diesel engines evolve, so too must the lubricants that protect them.

For example, federal regulatory measures have long spurred heavy-duty original equipment manufacturers (OEMs) to reengineer critical engine hardware in an effort to reduce emissions and enhance fuel efficiency. Specifically, today and in the coming years, OEMs are focused on a few key priorities, including:

• A 75% reduction in nitrous oxide (NOx) emissions during operation
• Lower particulate matter (PM) by roughly half
• Extended service life of aftertreatment systems from about 435,000 miles to 800,000 miles

API CL-4 and API FB-4 were developed with these shifts in mind and will help enable OEMs to meet these goals. Oils meeting the required performance are further engineered to deliver real value to fleet operators and other end users. And to seize that value, it’s important to match the right lubricant with the right application.

For example, in-warranty trucks should stick with fluids that have been specifically approved by the manufacturer or risk voiding the warranty on a critical asset. This is a point worth digging into further. It’s not uncommon for successful fleet operators to stick with what they know, and for many, a 15W-40 viscosity grade oil that meets current API standards has been a reliable choice. However, many OEMs allow for the application of lower-viscosity formulations—including 5W/10W-30 grades—that can contribute to enhanced fuel economy.

On a truck-to-truck basis, moving from a 15W-40 fluid to a 5W-30 fluid meeting contemporary API standards can lead to a roughly 0.5%-1.5% improvement in fuel economy, based on millions of miles of historical testing data compiled by Lubrizol. And while that may seem incremental, the potential fuel savings across an entire fleet can be significant.

The API FB-4 category can take these benefits even further. API CL-4 and API FB-4 are distinguished by a key difference in their viscosity chemistry. FB-4 fluids have lower high-temperature high-shear (HTHS) viscosity, specifically designed for modern, cutting-edge engine conditions. They are generally acceptable in engines manufactured in 2017 and newer, but end users should check with their engine suppliers for verification.

While it currently remains to be seen which heavy-duty diesel OEMs will approve the use of API FB-4 fluids, even better fuel economy gains can be expected. Historical data shows that API FA-4 fluids, the current low HTHS category and FB-4’s predecessor, can contribute to an expected 2% fuel economy increase.

And while extensive testing has shown that the risks of misapplying API FB-4 in older-model vehicles are minimal from a performance standpoint, it may void your manufacturer’s warranty. Remember: all fluids certified under either API CL-4 or API FB-4, no matter the viscosity grade, must meet the same performance requirements as outlined by the testing matrix described earlier in this article. There are no compromises to be made in terms of durability, engine protection, and overall high levels of performance.

Managing logistical concerns

In advance of the coming transition to API CL-4 and API FB-4 fluids, fleet managers may find themselves wondering how to best manage the switch from a logistical standpoint. The most helpful action fleets can take today is to begin early conversations with your oil suppliers on the transition.

Your suppliers should be able to help you more fully understand availability timelines, labeling differentiation, and suggest other things to look for.

For example, lubricants meeting the new API CL-4 standards are fundamentally different than their older-generation counterparts, an important point for fleets that regularly 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 to help fleets catch any issues before they become larger problems. But because API CL-4 will have a different baseline profile than API CK-4 fluids, it may in some instances contribute to new variations in the signature of your oil samples. Your oil supplier should be able to help you make sense of any variances.

If any portion of your fleet represents a good candidate for the potential application of API FB-4 fluids, your oil supplier should be able to help you identify these opportunities as well. In these instances, mixed-age fleets would need to stock two oil tanks at service sites, which has the potential to create some logistical complexity. Collaboration with your oil supplier should help you determine whether or not the potential benefits outweigh such concerns based on the composition of your fleet. In any case, proper training of your technicians on the applicability of your oils is essential, and your supplier may be able to assist you on this front as well.

The bottom line is that staying educated on the available technologies can help you make the best lubricant choices for your business and successfully manage the transition. API CL-4 and API FB-4 were developed specifically to enhance the potential performance of today’s and tomorrow’s heavy-duty diesel engines. It’s up to forward-thinking fleet managers to make the most of it.