Api Cl-4 Engine Oil Explained- The New Heavy-duty Diesel Oil Standard For 2027

By Author: bdean
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Heavy-duty diesel engines are entering a new generation — and the engine oils protecting them are evolving too.
API CL-4 is the new heavy-duty diesel engine oil category succeeding API CK-4. Developed through the industry program known as PC-12, it will be introduced alongside API FB-4, the successor to API FA-4.
The first licensing date for both API CL-4 and API FB-4 is January 1, 2027.
The new standards have been developed as engine manufacturers prepare for evolving engine technologies and emissions-control requirements. They introduce updated lubricant performance requirements in areas including oxidation control, wear protection, soot-related viscosity control and compatibility with modern emissions aftertreatment systems.
So, what is API CL-4? How does CL-4 differ from CK-4? And where does the new API FB-4 standard fit?
Here is what lubricant users, workshops, distributors and fleet operators need to know.
What Is the API CL-4 Engine Oil Standard?
API CL-4 is the next API C-category for heavy-duty diesel engine oils, succeeding API CK-4.
The specification was developed ...
... under PC-12 (Proposed Category 12), an industry program established to develop the next generation of heavy-duty diesel engine oil performance requirements.
PC-12 has resulted in two new API Service Categories:
API CL-4 — succeeds API CK-4
API FB-4 — succeeds API FA-4
This two-category structure follows the approach established by the previous PC-11 development program, which introduced CK-4 and FA-4.
The distinction is important because CL-4 and FB-4 address different viscosity and application requirements.

Why Is API CL-4 Replacing API CK-4?
Heavy-duty diesel engine technology continues to develop.
Modern engines and emissions-control systems are being engineered to meet increasingly demanding requirements for emissions, durability and efficiency. These changes can also influence operating temperatures, engine hardware and the demands placed on the lubricant.

Engine oil must therefore evolve alongside engine technology.

API CK-4 has served the heavy-duty market since its introduction in 2016. PC-12 updates the performance framework for the next generation, with API CL-4 becoming the successor to CK-4.

Rather than changing the fundamental purpose of heavy-duty engine oil, CL-4 builds on the established C-category approach with updated requirements appropriate for newer engine and aftertreatment technologies.

What Is PC-12?
PC-12 stands for Proposed Category 12.
It is the industry development program behind the new API CL-4 and API FB-4 heavy-duty diesel engine oil standards.

PC-12 was initiated in response to the changing requirements of heavy-duty engine manufacturers as they prepare new engine and emissions-control technologies.
Following industry development, testing and approval, CL-4 and FB-4 have been accepted into the API framework, with January 1, 2027 established as the first allowable licensing date.

From that date, qualifying oils may begin carrying API CL-4 or API FB-4 within the API Service Symbol in accordance with API licensing requirements.

API CL-4 vs CK-4: What Is Changing?
API CL-4 succeeds CK-4, but the transition should be understood as an evolution of heavy-duty engine oil performance rather than simply a change in name.
Important areas addressed through the PC-12 development program include:

Oxidation Control
Heavy-duty engine oils are continuously exposed to high temperatures during operation.
Over time, oxidation can contribute to oil degradation, viscosity increase and deposit formation.
Oxidation performance therefore remains an important part of the new CL-4 requirements as lubricant technology adapts to the operating environment of newer heavy-duty engines.

Wear Protection
Protecting critical engine components against wear remains one of the fundamental functions of a heavy-duty engine oil.
The PC-12 program incorporates performance requirements designed to ensure appropriate wear protection as engine hardware and operating conditions continue to evolve.

Soot and Viscosity Control
Soot management is particularly important in diesel engine lubrication.
Soot entering the lubricant can contribute to viscosity increase and affect engine oil performance.
The new specification includes updated testing and performance requirements addressing soot-related viscosity increase.

Emissions-System Compatibility
Modern diesel engines increasingly depend on sophisticated exhaust aftertreatment systems.
Because lubricant composition can influence these systems over time, the formulation of heavy-duty engine oils must balance engine protection with compatibility with emissions-control hardware.
PC-12 therefore places continued emphasis on lubricant chemistry and aftertreatment-system protection.

API CK-4 vs API CL-4

API CK-4 API CL-4
Development program PC-11 PC-12
Generation Current C-category Next C-category
Introduction 2016 First licensing January 1, 2027
Application Heavy-duty diesel engines Heavy-duty diesel engines
Category relationship Established category Successor to CK-4
Backward compatibility approach Broad application Designed to maintain C-category backward compatibility
Technology focus Current heavy-duty engine requirements Updated requirements for evolving engine and aftertreatment technology

For any vehicle or equipment, the engine manufacturer's recommended oil specification and viscosity grade should always be followed.

Is API CL-4 Backward Compatible With CK-4?
Backward compatibility is one of the important distinctions between the two new PC-12 categories.

API CL-4 has been developed as the backward-compatible side of PC-12, continuing the broad-application approach of the API C-category.

This is particularly relevant for fleets operating vehicles of different ages. A mixed fleet may contain newer engines alongside equipment currently operating with CK-4 lubricants.
However, lubricant selection should always be based on the engine manufacturer's requirements for the specific engine, application and viscosity grade.

What Is API FB-4 Engine Oil?

API FB-4 is the second new heavy-duty diesel engine oil category created through PC-12.

While CL-4 succeeds CK-4, FB-4 succeeds FA-4.
FB-4 addresses the industry's continued development of lower-viscosity heavy-duty engine oils for compatible newer engine platforms.
Lower-viscosity lubricants can play a role in reducing lubricant-related friction and supporting vehicle efficiency, but they must be used only where the engine manufacturer permits the relevant viscosity grade and performance category.
This makes understanding the distinction between API CL-4 and API FB-4 particularly important.

API CL-4 vs FB-4: What Is the Difference?
Although CL-4 and FB-4 were developed together under PC-12, they are not interchangeable categories.
Their primary distinction relates to viscosity and intended application.
API CL-4 continues the traditional C-category route and is designed around