Truck operators are seeing a major category change in heavy-duty diesel engine oil for the first time in nearly a decade. The American Petroleum Institute (API) will introduce PC-12 starting in January 2027, replacing the current CK-4 and FA-4 standards with two separate categories for different generations of engines.
“The change will impact all who purchase engine oil, from huge fleets that run hundreds of trucks to small and independent owner-operators. It will be important to understand the difference between the two new categories in advance of the next maintenance interval, especially for companies that are planning equipment acquisitions, maintenance budgets and fuel efficiency investments.
This is not just another label on top of an existing product. Categories of heavy-duty diesel engine oil are usually substantially revised about every decade. The last big changes were in 2016 and 2006, so the introduction of PC-12 is a big deal for the trucking industry.
Fleet executives should see this transition as an issue of procurement and equipment planning, not as routine maintenance modification. Bulk oil buying, maintenance specs, warranty requirements for trucks ordered for 2027 and later, and fuel economy calculations will require review. Owner-operators have to make the same decisions, only on a smaller scale. Often it is in selecting products at a parts counter or discussing an oil change at a repair shop.
Technical details also require close examination. Performance and longevity data need to be understood in context, particularly when test results are provided by manufacturers touting their own formulations. The analysis below uses technical information on the new categories, but separates manufacturer-reported findings from independently verified evidence.
PC-12 Introduces Two Different Oil Categories
Heavy-duty diesel engines currently rely primarily on two oil categories: CK-4 and FA-4. CK-4 is the widely used, backward-compatible standard designed for a broad range of older and newer engines. FA-4, by contrast, offers a lower-viscosity formulation intended to improve fuel economy in compatible modern engines.
PC-12 will replace both categories, but it will not introduce a single universal oil. Instead, it will establish two distinct specifications with different compatibility requirements.
The first is API CL-4, the successor to CK-4. Designed to remain backward compatible, CL-4 is intended for existing engines as well as newer models approved for the category. That includes trucks dating from 2015 and vehicles introduced in 2024, provided their manufacturers’ specifications permit its use.
CL-4 will continue to be available in familiar viscosity grades, including 15W-40, 10W-30 and 5W-30. It is expected to be the principal replacement for CK-4 across much of the existing heavy-duty fleet.
The second category is API FB-4, a lower-viscosity option developed to deliver improved fuel economy in engines specifically engineered for it. Unlike CL-4, FB-4 is not backward compatible. Its intended applications are model-year 2027 and newer engines designed around the new specification.
That distinction is critical. Operators cannot simply switch an existing truck to FB-4 in an attempt to reduce fuel consumption. An engine must be designed and approved for the formulation. Using an incompatible oil could compromise engine protection and expose an operator to expensive mechanical damage.
FB-4 will also introduce the XW-20 viscosity family, covering 0W-20 and 5W-20 oils. These grades are considerably thinner than the 10W-30 and 15W-40 formulations commonly used by fleets today. Their lower viscosity is central to the fuel-saving potential of the new category.
Emissions Rules Are Driving the Change
The arrival of PC-12 reflects the combined influence of tighter emissions requirements and changes in diesel-engine design.
Model-year 2027 emissions and greenhouse-gas standards are pushing engine manufacturers to develop cleaner, more efficient powertrains. The engines are being designed to reduce nitrogen oxide emissions by approximately 80% and particulate emissions by around 50% compared with earlier generations.
Meeting those targets requires more advanced engine designs, with tighter operating tolerances, higher temperatures and greater demands on lubrication. Engine manufacturers, working through their industry trade association, formally requested the development of a new oil category capable of meeting these requirements.
In other words, the engine technology is changing, and the lubricant must evolve alongside it.
Lower-viscosity oil can reduce the energy an engine consumes moving lubricant through its internal components. A useful comparison is the difference between moving honey and moving water: the thicker substance requires more effort, while the thinner one flows more easily. The same principle helps explain why XW-20 formulations can reduce internal friction and support better fuel economy in compatible engines.
However, reducing viscosity creates an engineering challenge. A thinner oil film provides less physical separation between moving metal surfaces, potentially increasing the demands placed on anti-wear additives.
At the same time, the new specifications tighten limits on sulfated ash, phosphorus and sulfur, collectively known as SAPS. These chemical components can contribute to deposits or damage in exhaust aftertreatment equipment, including diesel particulate filters (DPFs) and selective catalytic reduction (SCR) systems.
Reducing SAPS levels can help protect emissions-control hardware and support compliance with environmental requirements. Yet some of the same chemical components have traditionally contributed to engine wear protection. Formulators must therefore find ways to maintain durability while reducing their concentration.
To address that challenge, the new category introduces an engine test specifically targeting cylinder-liner scuffing. The test subjects engine components to demanding conditions intended to expose the severe liner and piston-ring wear that inadequate lubrication could cause. Oils must withstand a prescribed number of operating hours under those conditions to pass.
The addition is particularly relevant for operators concerned about the consequences of using thinner oil in modern diesel engines.
Will the New Formulations Make Engines Wear More?
A main worry about PC-12 is that it could shorten the life of the engine by reducing the traditional chemistry that prevents wear.
Under CK-4, concentrations of phosphorus averaged most often around 1,200 parts per million. In the new CL-4 category, the maximum level is cut to 800 parts per million and zinc levels are also reduced. This represents a major change in the chemistry used to protect engine components.
The makers of the new lubricant formulations say the reduction can be compensated for by adding more anti-wear additives. “We have not seen a correlating increase in wear in our field testing,” one manufacturer stated.
The company’s numbers show its trials spanned about 15 million miles in a wide array of applications. The list included long-haul trucking, heavy coal hauling, mining operations, agricultural equipment, diesel pickups, local delivery vehicles and city taxis.
The manufacturer also noted inspections of two Detroit Diesel DD15 engines in a North Carolina fleet. Both of the engines had over 700,000 miles on them. They started out on CK-4 oil before switching to the new PC-12 formulation and well over half their total mileage was run on the newer chemistry.
The company said the internal components were very clean after the engine was disassembled. It also included a main-bearing inspection that reportedly showed practically no measurable wear after more than 700,000 miles.
These are encouraging results, especially with worries about thinner oil and less phosphorus levels. However, an important caveat must be taken into account in interpreting these results: the manufacturer presented the results to promote its own product.
To get a broader picture of durability, independent long-term testing of several oil brands, engine models and operating conditions will be needed. Such evidence takes years to gather.
Thus the manufacturer’s results are an encouraging early pointer but cannot be taken as conclusive independent proof that all formulations will yield the same results under all operating conditions.
What fleet operators must do before January 2027
Once the difference between the two categories is understood, the real-world implications of PC-12 are pretty straightforward for trucking companies and independent operators.
1. Use the right oil for each engine.
CL-4 will replace CK-4 for most of today’s trucks. The system is backward compatible, so operators can make the transition without having to change their equipment simply because the oil category has been upgraded.
With CK-4 products beginning to move off the shelves, CL-4 is expected to become the primary replacement for compatible engines. The concept is to maintain existing maintenance procedures and not to impose an across-the-board change to service intervals.
2. Do not run FB-4 without engine confirmation.
This is the first safeguard. FB-4 is developed for engines that are specifically designed for the new, low-viscosity formulation, primarily model year 2027 and newer vehicles.
Using it in an older engine not approved for the class is not a valid shortcut to reduce fuel consumption. It could wear out and damage the engine.’
Where the switch to FB-4 is driven by price difference or fuel-economy claim, operators should verify the engine manufacturer’s requirements before purchase.
3. trucks already on the road will probably have an easier time.
Much of the manufacturer’s field data suggests a significant percentage of trucks will be able to transition from CK-4 to CL-4 with little disruption. New category is intended to be compatible with current service conditions and oil change intervals, provided the oil is approved by the engine manufacturer.
This means that for most operators the change should be in the selection of the right replacement product and not the introduction of a new maintenance regime. But in all cases this should be verified against the applicable engine specification.
4. Maintain and follow OEM maintenance guidelines.
The main reference for the selection of the oil viscosity and category are still the recommendations of the engine manufacturer in case of a specification change.
Fleet managers will have to check warranty requirements, make sure purchasing teams are ordering the right products and keep copies of invoices, service records and oil analysis results. If an engine problem turns into a warranty dispute, these documents could be very important.
Operators with existing oil analysis programs should also trend wear metals during the transition. Testing of used oil can provide valuable information about the performance of a formulation in a particular engine, duty cycle and operating environment.
5. Match fuel savings to real-world cost/mile
FB-4 and its viscosity grades XW-20 could offer significant fuel savings benefits to fleets operating model-year 2027 and newer trucks. What might seem to be a small cut in usage can add up to a lot of money over a year and hundreds of vehicles.
But these savings are predicated on the engine being designed to exploit the formulation. They should not be attributed to the oil alone.
Total cost per mile, projected fuel consumption and fuel-surcharge recovery should be considered by fleet managers when calculating. A half-cent-a-mile improvement might be a big annual win for a large carrier that replaces tractors every three years. But the value will be different for an owner-operator who keeps a fully paid-off truck for the long haul.
Why the PC-12 Transition Is Important
The introduction of PC-12 is the biggest change in heavy-duty diesel engine oil specifications in almost 10 years. The premise is simple: existing compatible engines will transition from CK-4 to CL-4, while approved 2027-and-newer engines will be able to use the thinner formulation, FB-4, which was developed to enable more fuel efficiency.
Operators’ immediate priority isn’t to hoard older oil or to scurry for a new buying strategy. It is knowing which specification applies to each truck and making sure each purchase meets the engine manufacturer’s requirements.
The difference between CL-4 and FB-4 is particularly important, as using the wrong formulation can turn an ordinary maintenance purchase into a potentially costly repair. Carriers that set clear purchasing rules before January 2027 will be better able to manage the transition without disruption.
The new standard also provides an opportunity to review equipment planning and operating costs. Fleet owners looking to purchase next-generation trucks should be keenly interested in the fuel savings potential of FB-4. For existing vehicles, the emphasis continues to be on maintaining engine protection, warranty requirements and reliable service.
Knowing this, before the new products hit the market, will make the transition easier for operators, rather than forcing them to make decisions at the parts counter without the information.
This article draws on a technical briefing from Shell Rotella’s formulation team at the Shell Technology Center Houston, published American Petroleum Institute category information, and emissions-standard guidance from the Environmental Protection Agency and the Truck and Engine Manufacturers Association. Field-performance figures and engine teardown findings are reported by the manufacturer and have not been presented as independently verified results.
