Diesel engine oil: how to check SAE viscosity and API category
Record the engine requirement for both the SAE viscosity grade and the API category before requesting a quotation. A matching viscosity number does not show that two diesel oils are interchangeable.
1. Keep the viscosity grade and the API category separate
An SAE grade such as 15W-40 describes viscosity. An API category such as CK-4 or CI-4 describes a performance classification. An engine manual may require both. Write them as separate items so a quotation can be checked against the request, not only against the number printed on the label.
2. Start from the engine requirement
Record the engine or vehicle model, the oil specification in the manual, and any required approval. If the manual is unavailable, ask the equipment supplier before treating the name of the oil now in use as the specification. A preferred brand is not the same as a required approval.
3. Do not treat CK-4 and FA-4 as substitutes
API describes CK-4 oils as exceeding the performance criteria of CJ-4, CI-4 with CI-4 PLUS, CI-4 and CH-4, and as able to lubricate engines calling for those categories. API also says that fuel sulfur above 15 ppm may affect aftertreatment durability or the oil-drain interval, and that the engine manufacturer should be consulted in that case.
API states that FA-4 oils are not interchangeable or backward compatible with CK-4, CJ-4, CI-4 with CI-4 PLUS, CI-4 and CH-4 oils. Confirm the engine manufacturer’s requirement before using an FA-4 oil. This guide does not assign a category to a particular engine.
4. Match the catalogue line to the quoted product
This site lists an EILONG CK-4 diesel engine oil option and a TRIOSD SAE 15W-40 diesel option. Confirm the SAE grade, API category, current documents and availability for the product that is actually quoted. A catalogue description is not a batch certificate or an engine approval.
When replacing or topping up, ask for compatibility guidance for the existing oil before use. Include the details below with the inquiry.
- Engine or vehicle model, plus the required SAE grade and API category
- Name of the oil now in use, when replacing or topping up
- Pack size, quantity and destination
- Request the current TDS and SDS for the exact grade being quoted
Questions about this topic
Can I order 15W-40 when the manual only says diesel engine oil?
Not from that description alone. Ask for the required SAE grade and API category, or share the engine model so both can be checked before a product is offered.
Is CK-4 always suitable where an older CI-4 oil was used?
Do not decide from the category names alone. API describes CK-4 as exceeding the criteria of several earlier diesel categories, but the engine manufacturer, fuel sulfur and any extra approval still need to be checked. FA-4 is a separate category and is not a substitute for CK-4.
