PC-12 Diesel Oil: API CL-4 & FB-4
PC-12 is the industry development program behind the next heavy-duty diesel engine-oil categories: API CL-4 and API FB-4. The categories are now published in API 1509, with first licensing beginning January 1, 2027.
Last technical review: August 20, 2026 · API 1509 24th Edition + current OEM service documentation
CL-4 is the backward-compatible side of the new standard and is designed to meet or exceed CK-4 performance. FB-4 is not universally backward compatible. It includes selected xW-30 and new xW-20 heavy-duty oils with lower HTHS viscosity. Do not put FB-4 into an engine unless the engine manufacturer specifically allows the category and viscosity.
What Is PC-12 Diesel Oil?
PC-12 means Proposed Category 12. It was the development name used by the heavy-duty engine-oil industry while creating the specifications that become API CL-4 and API FB-4.
The terminology matters. PC-12 is the development program. CL-4 and FB-4 are the API service categories that oil buyers will eventually see on licensed products.
Two Categories — Two Different Jobs
API CL-4
CL-4 is the traditional heavy-duty C-category side of PC-12. API states that CL-4 oils are designed to meet or exceed the performance criteria of CK-4, CJ-4, CI-4 and CH-4.
- Designed for 2027 heavy-duty diesel engine service
- Backward-compatible performance path
- HTHS minimum remains 3.5 cP for xW-30
- Supports modern DPF and aftertreatment durability
- Likely broad mixed-fleet relevance
API FB-4
FB-4 is the lower-viscosity category. API defines it for certain xW-30 and xW-20 oils intended for select engines designed around the 2027 emissions era.
- HTHS range of 2.6–3.2 cP
- Includes xW-20 heavy-duty diesel oils
- Designed partly to support lower greenhouse-gas emissions
- Not interchangeable with CK-4 by default
- Requires engine-manufacturer compatibility
API specifically states that FB-4 oils are not interchangeable or backward compatible with CK-4, CJ-4, CI-4 and CH-4 oils. Check the engine manufacturer's oil requirement before using FB-4.
What Does My Engine Require Right Now?
PC-12 does not erase the oil specification already attached to your engine. Until an engine manufacturer publishes a new requirement for a specific model and model year, continue using the currently approved oil specification and viscosity.
| Engine | Current Requirement | Typical Current Grade | CL-4 / FB-4 Status |
|---|---|---|---|
| Cummins X15 | Cummins CES 20086 / API CK-4 for current X15 applications. | 10W-30 or 15W-40 depending application, temperature and manual. | Continue CES 20086 requirements until Cummins publishes the applicable PC-12-era requirement. |
| Detroit DD15 | DFS 93K222 / API CK-4. DFS 93K223 / API FA-4 is also permitted in applicable Detroit heavy-duty engines. | Application and Detroit approval dependent. | Do not assume FB-4 compatibility until Detroit identifies approved applications. |
| Volvo D13 | 2021+ recommended: VDS-5.0 / API FA-4. Minimum: VDS-4.5 / API CK-4. 2017–2020: VDS-4.5 / CK-4. | 10W-30 is common in current Volvo documentation. | Future CL-4/FB-4 use must follow Volvo's model-year-specific requirements. |
| Mack MP8 | Mack EOS-4.5 premium oil for current service guidance. | Application and ambient-temperature dependent. | Continue EOS-4.5 requirements until Mack publishes applicable new specifications. |
| PACCAR MX-13 | API CK-4 for EPA 2017, EPA 2021 and EPA 2024 MX-13 engines. | SAE 10W-30 CK-4 is PACCAR's primary recommendation; other approved CK-4 grades may be used within the viscosity chart. | Use CK-4 until PACCAR publishes model-specific CL-4 or FB-4 guidance. |
Why list the current specification? Because that is the specification the truck needs today. PC-12 is important, but an owner/operator should not change oil simply because a newer API category exists.
5W-20 Oil in a Class 8 Diesel?
Yes. FB-4 creates a path for xW-20 heavy-duty diesel oils. That does not mean a bottle of 5W-20 belongs in an X15, DD15, D13, MP8 or MX-13 unless the engine manufacturer approves it.
Traditional C-Category
CL-4 xW-30 oils retain an HTHS viscosity requirement of at least 3.5 cP.
FB-4
API defines FB-4 oils with an HTHS range of 2.6–3.2 cP, allowing both xW-30 and xW-20 formulations.
HTHS — high-temperature/high-shear viscosity — is important because engine bearings and other highly loaded components do not operate under the same conditions as an oil sitting in a bottle at room temperature. The engine has to be designed and approved for the lubricant's operating viscosity.
Lower viscosity can reduce pumping and friction losses, but the engine manufacturer's approved category, viscosity and proprietary specification come first.
PC-12 Tightens the Chemical Box
Modern diesel oil has to protect the engine while also protecting increasingly expensive emissions hardware. PC-12 tightens limits on sulfated ash, phosphorus and sulfur compared with CK-4.
| Property | API CK-4 | API CL-4 / PC-12 |
|---|---|---|
| Sulfated Ash | 1.0% max | 0.9% max |
| Phosphorus | 0.12% max | 0.08% max |
| Sulfur | 0.4% max | 0.3% max |
This is not simply a matter of removing additives. Phosphorus-containing anti-wear chemistry has traditionally played an important role in protecting loaded engine components. The formulation still has to pass demanding wear, oxidation, soot and aftertreatment requirements while staying inside the tighter limits.
What Is PC-12 Trying to Improve?
Oxidation
PC-12 includes tougher oxidation control requirements to handle hotter, more demanding modern engine operation.
Wear & Scuffing
The specification includes modern wear testing, including Detroit DD13 scuffing performance.
Aftertreatment
Lower SAPS limits help support durability of DPF and other advanced emissions-control systems.
Soot Control
Updated testing evaluates the oil's ability to control viscosity increase caused by soot contamination.
Fuel Economy
FB-4 provides lower HTHS options intended for engines designed to take advantage of reduced viscous losses.
2027 Hardware
The specification is designed around the performance needs of the next generation of heavy-duty diesel engines.
Does CK-4 Disappear in 2027?
No. The arrival of CL-4 does not mean every truck currently using CK-4 suddenly requires a different oil.
API CL-4 is designed to meet or exceed CK-4 performance, but CK-4 remains relevant to millions of engines already in service. Fleets should continue matching lubricant selection to the engine manufacturer's specification, model year, viscosity requirement and duty cycle.
For a mixed fleet, the best bulk-oil decision is not automatically the newest category. The objective is to select an oil that legitimately covers every engine intended to use that bulk tank.
What Fleets Should Do Before 2027
- Inventory every engine model and model year in the fleet.
- Record the current API category and OEM oil specification for each engine.
- Do not assume FB-4 is compatible with older engines.
- Watch Cummins, Detroit, Volvo, Mack and PACCAR service publications for updated requirements.
- Confirm that any bulk oil covers every engine connected to that bulk tank.
- Keep oil-analysis and maintenance records if extending drain intervals.
- Do not extend oil drains based only on the fact that an oil is synthetic.
What Is Confirmed — And What Are We Still Watching?
Confirmed in API 1509, 24th Edition.
Confirmed in API 1509, 24th Edition.
Confirmed by API.
API states CL-4 is designed to meet or exceed CK-4, CJ-4, CI-4 and CH-4 performance criteria.
Continue checking Cummins, Detroit, Volvo, Mack and PACCAR documentation as new engines and approvals are published.
Product-specific claims should not be made until the oil marketer publishes them.
PC-12 Oil Questions
What does PC-12 stand for?
PC-12 means Proposed Category 12. It was the development program for the heavy-duty diesel oil specifications that become API CL-4 and API FB-4.
When does PC-12 take effect?
API CL-4 and FB-4 first licensing begins January 1, 2027.
Does CL-4 replace CK-4?
CL-4 becomes the newest API C-category and is designed to meet or exceed CK-4 performance. That does not mean every existing CK-4 engine suddenly requires a different oil.
Is CL-4 backward compatible with CK-4?
Yes. API states that CL-4 oils are designed to meet or exceed the performance criteria of CK-4, CJ-4, CI-4 and CH-4.
Is FB-4 backward compatible?
No, not universally. API specifically states that FB-4 oils are not interchangeable or backward compatible with CK-4, CJ-4, CI-4 and CH-4. Follow the engine manufacturer's recommendation.
Will there really be 5W-20 diesel oil for semi trucks?
FB-4 permits certain xW-20 heavy-duty diesel oils. Use one only in an engine specifically approved for that API category and viscosity.
What oil does my Cummins X15 require now?
Current X15 applications commonly require oil meeting Cummins CES 20086 / API CK-4. Cummins documentation commonly lists 10W-30 or 15W-40 depending on application and operating temperature. Check the exact engine manual before changing category or viscosity.
Should I change from CK-4 as soon as CL-4 comes out?
Not simply because CL-4 is newer. Use the API category, OEM specification and viscosity approved for the exact engine.
Technical Verification
This page is maintained as a living specification reference. API and engine-manufacturer documentation take priority over marketing claims and third-party summaries.