API CK-4 vs CI-4: Diesel Oil Selection for Fleet Buyers
What CK-4 coverage of CI-4 means—and the separate viscosity, OEM, fuel and exact-product checks a fleet buyer still needs.
API CK-4 can meet an API CI-4 service-category requirement, according to API. That answers the category comparison; it does not approve every CK-4 product for every engine. A fleet purchase still has to match the required SAE viscosity, engine-maker specification and operating conditions. Do not reverse that logic and assume CI-4 satisfies a CK-4 requirement.
The MOS CK-4/CI-4 diesel-engine-oil Product page lists both performance-level options in one catalogue line. Treat it as a starting point for a grade-specific enquiry, not proof that all listed viscosities or private-label offers hold the same licence or OEM approval.
Separate API category from SAE viscosity
Write three separate entries in the purchase specification: the API service category, the SAE viscosity grade required for the actual engine, and any applicable OEM specification or approval. None can be substituted for another. Two oils with the same SAE label need not have the same performance category; two oils naming CK-4 need not both satisfy a particular OEM requirement.
Copy the engine requirement from the relevant manual or current service specification before comparing an offer. If the proposed change also alters viscosity, brand or product identity, record those as separate changes. A newer category name is not permission to change the viscosity specified for the engine.
Where CK-4 and CI-4 differ
API currently lists both CK-4 and CI-4 as current categories. Their design contexts and performance requirements differ. The following summary uses API’s official Oil Categories guidance; the emission-year descriptions are category context, not an application approval or a regulatory-status claim for a MOS product.
| Decision | CK-4 | CI-4 | Buyer check |
|---|---|---|---|
| Design context | High-speed four-stroke diesel service associated with 2017 on-highway and Tier 4 non-road emission requirements, including earlier model years. | High-speed four-stroke diesel service associated with 2004 emission requirements implemented in 2002; includes EGR operating conditions. | Identify the actual engine requirement rather than select only by model year. |
| Earlier-category coverage | API states CK-4 exceeds CI-4 performance criteria and can lubricate engines calling for CI-4. | A CI-4 designation does not establish CK-4 performance. | Retain the required SAE grade and check the applicable OEM specification and conditions. |
| Emissions equipment and fuel | Category guidance addresses particulate filters and other advanced aftertreatment, with a warning about higher-sulfur fuel and maintenance. | EGR context does not establish suitability for every later aftertreatment system. | Record actual emissions equipment and fuel; obtain applicable engine-maker guidance. |
| Evidence for the offer | Exact brand, product and SAE grade, with documents supporting the claims actually made. | The same exact-product verification. | A family catalogue name is not independent proof of an API licence or OEM approval. |
Check OEM, aftertreatment and fuel before changing
A particulate filter in the fleet is a reason to check the engine-specific lubricant requirement, not to approve any bottle labelled CK-4. Match the exact offered grade to that requirement and the relevant supporting evidence. Likewise, a CI-4 category description that mentions EGR does not by itself approve the oil for a different emissions system.
API warns that using CK-4 with fuel sulfur above 15 ppm may affect aftertreatment durability and/or the oil drain interval, and directs users to the engine manufacturer for interval recommendations. This is not permission to use fuel that the engine prohibits. Record the actual fuel specification and operating conditions; do not turn a CK-4 label into a longer-drain commitment.
Keep FA-4 separate from this comparison. API explicitly says FA-4 oils are not interchangeable or backward compatible with CK-4 and CI-4 categories. An apparently similar viscosity label does not remove that boundary. Follow the engine manufacturer’s requirements instead of assuming that every newer diesel category is a replacement.
Resolve the specification and approval questions before any service fill or equipment trial. For an authorized change, retain the current oil identity, engine condition and maintenance baseline, and obtain the applicable changeover and interval instructions. A single satisfactory observation cannot establish universal compatibility or justify an unapproved interval extension.
Send an engine-specific diesel-oil RFQ
For a diesel-engine-oil grade enquiry, provide the engine or vehicle models, OEM requirement, required SAE viscosity and API category, current oil, emissions/aftertreatment equipment, fuel information and duty cycle. State the proposed change and the maintenance interval being considered, together with sample and order quantity, annual demand, packaging and destination.
Request current exact-grade TDS/SDS and relevant supporting performance or approval documents. If an API licence is asserted, verify the marketed brand, product and SAE grade through the API EOLCS licence-directory route. A company name or another product’s licence is not evidence for the oil in your quotation. Ask the supplier to identify what is supported for the actual offer; no licence, approval, fixed packaging, availability or drain-life promise is established by this comparison.