- What is PC-12?
PC-12, or Proposed Category 12, is the industry development program that produced the next heavy-duty diesel engine oil categories: API CL-4 and API FB-4. PC-12 is the industry development work that takes place behind the scenes before the new API CL-4 engine oil category is introduced to the market. - Is PC-12 simply a name change?
No. PC-12 changes performance requirements, chemical limits, and engine-test requirements used to evaluate heavy-duty diesel engine oil. - Why was PC-12 developed?
Diesel engine hardware, aftertreatment systems, and operating requirements continue to evolve. The new specifications were designed around improved oxidation resistance, enhanced wear protection, expanded elastomer compatibility, improved aftertreatment-system protection, and support for lower-viscosity grades aimed at fuel-economy strategies. - When will API CL-4 and API FB-4 first be introduced?
First licensing begins January 1, 2027. That is when qualified products/marketers can begin carrying the new API category designations. - Does my fleet need to convert to the new oil category on January 1, 2027?
No. January 1, 2027 is when manufacturers can begin licensing and labeling oils under the new API category. Fleets can transition based on engine requirements, product availability, and their normal maintenance schedules
API Diesel Engine Oil Evolution
From Early Engine Protection to PC-12, API CL-4 and API FB-4
Diesel engine oil history is more than a sequence of letters. Each API category reflects a change in engine design, emissions-control technology, operating severity, or fuel-efficiency strategy. Understanding that progression makes the next transition easier and helps explain why the newest category isn't automatically the right choice for every engine.
The American Petroleum Institute’s PC-12 development effort produced two new heavy-duty diesel engine oil categories: API CL-4 and API FB-4. First licensing begins January 1, 2027. CL-4 follows the traditional C-category path, while FB-4 is the lower-viscosity F-category path for specifically approved engines.
Why API Diesel Engine Oil Categories Change
Engine oil must operate inside the engine technology of its time. As diesel engines gained horsepower, turbocharging, exhaust gas recirculation, diesel particulate filters, selective catalytic reduction, and advanced aftertreatment systems, oil performance requirements had to advance as well.
The progression moved from basic engine cleanliness and wear protection to a broader system view that includes:
- Soot control and viscosity management
- Oxidation resistance under higher temperatures
- Deposit protection and oil-film durability
- Compatibility with diesel particulate filters and other aftertreatment systems
- Aeration and shear-stability performance
- Lower-viscosity options for fuel-efficiency strategies in approved engines
Era 1: Protect the Engine
CA through CE
Early API diesel classifications focused on keeping mechanically demanding diesel engines clean and protected. As power output, supercharging, and turbocharging increased, lubricant performance had to improve as well.
- Bearing-corrosion protection
- Piston and ring-deposit control
- Oxidation resistance
- Wear protection
- High-temperature stability
- Protection under increasing turbocharger severity
In this era, the lubricant conversation was centered mainly on the engine itself. Modern exhaust aftertreatment compatibility was not yet the dominate formulation constraint.
Era 2: CF-4 and CG-4
CF-4: High-output four-stroke diesels
CF-4 marked the transition to modern high-speed, four-stroke, heavy-duty diesel oil. Engine output and thermal loading were increasing, placing greater emphasis on piston cleanliness, deposits, wear, and oil consumption.
CG-4: Emissions and soot become major drivers
By the mid-1990s, changing combustion strategies and emissions requirements increased the importance of soot control. The oil was being asked not only to lubricate, but also to suspend combustion contaminants and resist viscosity increase and deposits.
CAUSE & EFFECT: Emissions-driven combustion changes = more demanding soot environment = stronger dispersancy, oxidation control and deposit constraint.
Era 3: CH-4 and CI-4
CH-4: The soot-control step
CH-4 was developed for engines meeting 1998 exhaust-emission requirements. Soot management became an increasingly important part of heavy-duty diesel lubricant performance.
Excessive soot loading can contribute to:
- Viscosity increase
- Abrasive wear
- Sludge and deposits
- Ring-zone contamination
- Heavy demand on the dispersant system
CI-4: The EGR era
CI-4 was a major step because cooled exhaust gas recirculation (EGR) became an important diesel emissions strategy. Returning some exhaust gas to the intake helped reduce NOx formation, but it created a tougher lubricant environment.
- Greater soot-contamination challenge
- Higher demand on dispersancy
- Stronger oxidation resistance requirements
- Soot-related wear control
- Greater emphasis on viscosity control and oil durability
REMEMBER: CI-4 = EGR. The immediate connection should be the increased lubricant stress associated with EGR-equipped heavy-duty diesel engines.
Era 4: CJ-4 and the Aftermarket Revolution
CJ-4 changed the lubricant mission. The oil now had to protect both the engine and sensitive exhaust aftertreatment systems used in the 2007-model-year emissions era.
The DPF Challenge
A diesel particulate filter (DPF) captures particulate matter from the exhaust. Normal engine oil consumption can introduce inorganic ash into the exhaust. Unlike combustible soot, this ash can accumulate in the DPF over time.
SYSTEM VIEW: Oil consumption = additive-derived ash enters exhaust = ash accumulates in the DPF = aftertreatment service becomes part of the lubricant conversation.
SAPS
SAPS is a common shorthand for sulfated ash, phosphorus and sulfur. CJ-4-era formulation placed greater emphasis on controlled chemistry and aftertreatment compatibility while still protecting the engine under severe diesel service.
ULSD
Ultra-low sulfur diesel (ULSD) was an important enabling element in the modern emissions-control environment and accompanied the transition toward DPF-equipped on-highway engines.
REMEMBER: CJ-4 = DPF / aftertreatment compatibility. This is the point where engine oil selection clearly becomes part of emissions-system protection.
Era 5: CK-4 and FA-4
For the 2017-model-year era, the API heavy-duty engine oil path split into two categories. CK-4 continued the broadly applicable traditional performance path, while FA-4 introduced lower-HTHS (High Temp High Shear test) oils for certain newer engines where the manufacturer permits their use.
CK-4: Modern durability
- Improved oxidation resistance for hotter, more highly stressed engines
- Improved aeration control
- Improved shear stability and viscosity retention
- Strong deposit protection
- Designed for modern emissions-equipped heavy-duty engines while retaining broad older-engine applicability when allowed by the manufacturer
REMEMBER: CK-4 = durability, oxidation, shear stability and modern engine protection.
FA-4: Fuel economy becomes part of the specification
FA-4 was developed for certain XW-30 oils used in select newer high-speed four-stroke diesel engines. Its lower high-temperature high-shear (HTHS) viscosity can help reduce hydrodynamic friction, supporting fuel-efficiency strategies in engines designed for it.
Era 6: PC-12, CL-4 and FB-4
PC-12: The next heavy-duty engine oil generation
PC-12 means Proposed Category 12. It is the heavy-duty diesel engine oil development program that produced two new API service categories: CL-4 and FB-4. The categories have been accepted for inclusion in API 1509, with first licensing scheduled for January 1, 2027.
Why PC-12 Was Needed
- Support next-generation heavy-duty diesel engine designs and 2027-era requirements
- Improve oxidation resistance and wear protection as engine operating severity continues to rise
- Expand elastomer and material compatibility requirements
- Strengthen protection of modern exhaust aftertreatment systems
- Create a path for lower-viscosity heavy-duty oils that can support fuel-economy strategies where the engine manufacturer approves their use
CL-4: The backward-compatible path
API CL-4 is the C-category side of PC-12. It follows the traditional heavy-duty diesel path and is intended to provide a new performance level above CK-4 while retaining backward-compatible application for earlier C-category service where the engine manufacturer allows it. For mixed fleets, CL-4 is expected to be the primary PC-12 category to evaluate.
FB-4: The lower-viscosity path
API FB-4 is the fuel-efficiency-oriented side of PC-12. It extends the F-category concept beyond FA-4 and is intended for select newer engines designed for lower-viscosity oils. It is not a universal replacement for CK-4, CL-4, or older C-category oils. Engine-manufacturer approval, viscosity grade, and application requirements must be verified before use.
TIMING: As of September 2026, CL-4 and FB-4 have been approved for incorporation into API 1509, but first licensing is scheduled for January 1, 2027. Until then, verify the currently licensed category required by the engine manufacturer.
API Diesel Engine Oil Evolution at a Glance
| Category | Technology | Primary Lubricant Challenge |
|---|---|---|
| CA / CB / CC (Obsolete) | Early diesel era | Basic deposit, oxidation and bearing-corrosion control |
| CD / CE (Obsolete) | Severe-duty and turbocharged diesels | Stronger wear, deposit and high-temperature protection |
| CF-4 (Obsolete) | 1990-era high-speed 4-stroke diesels | Improved piston cleanliness and oil-consumption control. Not suitable for most diesel engines built after 2009 |
| CG-4 (Obsolete) | Early emissions era | Greater soot, oxidation, and deposit control. Not suitable for most diesel engines built after 2009 |
| CH-4 | 1998 emissions era | Stronger soot handling and engine cleanliness |
| CI-4 / CI-4 PLUS | EGR era | Developed for high-speed, four-stroke diesel engines using EGR, providing enhanced engine durability and protection while operating on diesel fuel containing up to 0.5% sulfur by weight |
| CJ-4 | DPF + ULSD era | Developed for high-speed, four-stroke diesel engines meeting 2010 on-highway and Tier 4 non-road emissions standards. Provides improved engine protection and aftertreatment compatibility; designed for fuels up to 500 ppm sulfur; backward compatible with CI-4 PLUS, CI-4, CH-4, CG-4, and CF-4 |
| CK-4 | Modern severe-duty era | Developed for 2017 on-highway and Tier 4 non-road diesel engines. Enhanced protection against oxidation, shear-related viscosity loss, aeration, wear, deposits, soot-related viscosity increase, and aftertreatment system degradation; backward compatible with CJ-4, CI-4 PLUS, CI-4, and CH-4 |
| FA-4 | Fuel-economy path | Lower HTHS viscosity for approved newer high-speed diesel engines. Not interchangeable or backward compatible with CK-4, CJ-4, CI-4 PLUS, CI-4, or CH-4 |
| PC-12 - CL-4/FB-4 | 2027 next-generation era | Higher durability and wear demands, material compatibility, aftertreatment protection, and lower-viscosity efficiency options |
Technology-to-Lubricant Connection
| Engine/ Fleet Change | What Changes | Lubricant Response |
|---|---|---|
| Turbocharging | Higher temperature and deposit stress | Thermal/oxidation stability and deposit control |
| Higher horsepower | Greater load and power density | Wear protection and strong oil-film performance |
| Longer drain expectations | More time in severe service | Oxidation resistance, reserve alkalinity and durability |
| EGR | More severe soot/contaminant environment | Dispersancy, soot control and wear protection |
| DPF | Sensitivity to inorganic ash accumulation | Controlled SAPS and aftertreatment compatibility |
| Higher piston/turbo temperatures | Greater thermal load | Oxidation and deposit control |
| Fuel-economy pressure | Need to reduce parasitic losses | Lower viscosity where engine design allows |
| Mixed-age fleet | Different engine requirements | Correct category, viscosity, and backward-compatibility review |
PC-12 Frequently Asked Questions
API CL-4, API FB-4 and the Next Diesel Engine Oil Transition
Heavy Duty Engine Oil Performance Changes
Why? New engines demand more from the oil. The new category raises the performance standard to keep pace with engine technology and provide the protection, efficiency and reliability today’s equipment requires.
What do you need to do? Not much. Continue following your engine manufacturer’s oil recommendations. As the new category enters the market, make sure the oil you purchase carries the correct performance category and viscosity for your equipment. Your lubricant supplier can help simplify the transition, especially if you operate a mixed fleet.
- What is API CL-4 and Why?
API CL-4 is the new C-category and the successor to API CK-4. It follows the traditional heavy-duty diesel performance path and is designed for backward-compatible applications where the engine manufacturer, viscosity, and OEM requirements allow it. Used in 2027 and older heavy-duty diesel engines where API C-category oils are recommended. CL-4 is backward compatible, making it a good fit for mixed fleets\ - What is API FB-4 and Why?
API FB-4 is the new F-category and the successor to API FA-4. It is intended for select newer engines designed for lower HTHS-viscosity oils. API FB-4 is designed for newer heavy-duty diesel engines that can take advantage of lower-viscosity oils to improve fuel efficiency while still providing the protection required by modern engines. - What is the main difference between CL-4 and FB-4?
CL-4 is the broad C-category path designed for backward compatibility. FB-4 is the lower-viscosity, fuel-economy-focused, F-category path for specifically approved newer engines. The categories are not interchangeable in every engine. - Is CL-4 backward-compatible with CK-4 applications?
CL-4 is designed as the backward-compatible C-category path. The correct viscosity and any engine-manufacturer or OEM requirements must still be confirmed before changing products. - Is FB-4 backward-compatible?
Do not treat FB-4 as universally backward-compatible. Use FB-4 only when the engine manufacturer approves the category and viscosity. Should only be used where the engine manufacturer specifically approves the category and viscosity. - Will CL-4 and FB-4 immediately replace CK-4 and FA-4 everywhere?
No. CL-4 and FB-4 become the newest API C- and F-categories, but actual product and customer transitions will occur at different times based on equipment requirements, product availability and operating needs.
- What performance areas are changing?
The new specifications strengthen or update requirements involving oxidation resistance, wear protection, elastomer compatibility, aftertreatment-system protection, and lower-viscosity grades. PC-12 also updates the engine tests used to evaluate soot-related viscosity growth and piston-and-liner scuffing. - What does improved oxidation resistance mean?
Oxidation can contribute to oil thickening, deposits, acid formation, and loss of oil performance. Stronger oxidation requirements provide a more demanding evaluation of an oil under severe heat and operating conditions. - How is wear protection evaluated?
PC-12 includes a Detroit DD13 scuffing test that evaluates piston-and-liner adhesive wear. It adds another performance check for protection under scuffing conditions. - Why does soot-related viscosity control matter?
Soot can increase oil viscosity, contribute to deposits and increase wear. PC-12 uses an updated Cummins ISB viscosity test to evaluate soot-induced viscosity growth. - Why are phosphorus, sulfur, and sulfated ash controlled?
These components contribute to critical lubricant performance, but excessive levels can negatively affect emissions-system durability. Controlling them provides the optimum balance between engine protection and long-term DPF and aftertreatment performance. - Does PC-12 include additional seal-compatibility requirements?
Yes. The new specifications expand elastomer compatibility requirements, adding another material-compatibility consideration to the category. - Will PC-12 improve fuel economy?
The FB-4 category supports lower-viscosity options designed for fuel-economy strategies in approved engines. Actual fuel-economy improvement will vary based on engine design, duty cycle, operating conditions, viscosity grade, and other vehicle factors. - Does PC-12 automatically allow longer oil-drain intervals?
No. The category alone does not establish a longer drain interval for every engine or product. Follow the engine manufacturer’s interval, product guidance and an approved used oil analysis program when evaluating drain practices.
- Will off-highway equipment use CL-4?
C-category oils are used across many on-road and off-highway applications, but manufacturer positions and adoption timing may differ. Follow the equipment manufacturer’s published recommendation for specific machines or engines. - What about diesel pickup trucks?
Use the engine manufacturer’s required API category, viscosity grade, and OEM specification. - Can one CL-4 oil cover a mixed fleet?
CL-4 backward compatibility may create consolidation opportunities, but every engine’s category, viscosity, and OEM requirements must be checked before consolidating products. - Can FB-4 be used wherever CL-4, CK-4 or FA-4 is used?
No. FB-4 is not a universal replacement for C-category oils. It should be used only in engines that approve the F-category and viscosity. - Can CK-4 and CL-4 be mixed or commingled?
Do not assume compatibility or create a tank-transition procedure without product-specific guidance. Confirm the current product, replacement product, and approved transition recommendation with the lubricant provider.
- Why does the choice of lubricant provider matter during the PC-12 transition?
The transition may involve more than a product swap. A mixed fleet may require multiple oils, separate storage, updated labels, different dispensing practices, technician education, and ongoing technical support. Working with a knowledgeable provider helps ensure the transition is controlled and complete. - What should I expect a capable lubricant provider to help with?
A capable provider should help you evaluate your equipment population, confirm OEM requirements, develop a product and inventory plan, review storage and dispensing practices, and offer education and ongoing technical support. - Why is local support valuable?
A local provider can connect the technical recommendation with your actual equipment, inventory, storage, and operating practices. That context helps turn a specification change into a practical, controlled transition plan that fits your operation.
- Will used oil analysis results look different after changing formulations?
They may. Additive chemistry can differ between formulations, so elemental results and baselines may change. Record the product change, compare results with the correct product reference, and communicate the transition to your oil-analysis provider. With used oil analysis, you are testing and not just guessing.