API CI-4 vs CH-4 vs CK-4: Diesel Oil Ratings Explained

Next to the viscosity (like 15W-40), every diesel engine oil carries an API service rating such as CH-4, CI-4 or CK-4. Buyers often ignore it — but it tells you how modern the oil is and how well it handles soot, heat and wear. Here is what the letters mean and which one your engine needs.

How API diesel ratings work

API splits engine oils into two families: ‘S’ for petrol (spark) and ‘C’ for diesel (compression). The second letter advances through the alphabet as standards get tougher — so a ‘higher’ letter is a newer, more capable category. CH-4, CI-4 and CK-4 are three steps in that diesel progression.

The steps in between: CI-4 Plus, CJ-4 and FA-4

Between CI-4 and CK-4 there are two more categories you will see on packs and in manuals, and alongside CK-4 sits a parallel category that is easy to confuse with it. Categories from CG-4 and older are now obsolete.

The two numbers that matter most in this table are the jump at CJ-4, where chemical limits arrive for engines with filters and catalysts, and the FA-4 line. FA-4 oils have a lower high-temperature, high-shear viscosity to save fuel and must only go into engines whose maker specifies them. If you see FA-4 on a pack, do not treat it as a CK-4.

How to read the rating on the pack

API-licensed oils carry the API service symbol, often called the donut. The top half shows the service category, such as ‘API SERVICE CK-4’ or ‘CI-4/SL’ where an oil also covers a petrol category. The centre shows the SAE viscosity grade. When a pack lists several categories, the first one is the oil’s primary design.

• Licensed vs ‘meets the requirements of’. A licensed oil has been registered with API. ‘Meets’ or ‘suitable for’ is a supplier claim. Both are common in India; ask the supplier to show the category on the product data sheet.
• One category, one grade. ‘CI-4 15W-40’ is a complete description. A pack that says only ‘heavy-duty diesel oil’ without a category is not enough to buy on.
• Batch and date. Check the batch number and manufacturing date on drums and pails, and store them sealed, under cover and on their side so water cannot collect on the lid.

Are they backward compatible?

Generally, a newer category can be used where an older one is specified if the viscosity grade matches — for example CK-4 15W-40 in an engine that called for CI-4 15W-40. Newer oils simply offer more protection. The exceptions are some older or specialised engines, so always cross-check the owner’s manual before ‘upgrading’ a rating.

BS-VI after-treatment and why low-SAPS matters

Since April 2020, new Indian vehicles have been built to BS-VI and run on diesel with no more than 10 ppm sulphur. To meet the emission limits, most BS-VI diesels use some combination of exhaust gas recirculation (EGR), a diesel oxidation catalyst (DOC), a diesel particulate filter (DPF) and selective catalytic reduction (SCR) dosed with diesel exhaust fluid. Every one of these is sensitive to what the engine oil leaves behind.

• Sulphated ash comes from the metal-based detergent and anti-wear additives. A small amount of oil is always burned, and the ash it leaves cannot be burned off during DPF regeneration. It collects in the filter, raises back-pressure and brings forward filter cleaning.
• Phosphorus from anti-wear additives can coat and slowly poison the catalysts.
• Sulphur adds to catalyst stress and acid formation.

That is why oils for these engines are called low-SAPS (low sulphated ash, phosphorus and sulphur). CJ-4 and CK-4 cap all three; CH-4 and CI-4 have no such limits and are typically higher in ash. A CI-4 oil may protect the engine itself perfectly well, but in a DPF-equipped BS-VI engine it can shorten filter life — which is why the manual’s category, not just the viscosity, decides.

Tractors and gensets follow their own emission rules. Many tractors in service still use older-technology engines, while newer gensets built to CPCB IV+ norms may carry after-treatment. Treat each machine by its own manual.

Mixed fleets: old and new engines in one yard

Many Indian operators run BS-III and BS-IV trucks, newer BS-VI vehicles, tractors and a genset from the same store. Stocking a different oil for each is costly and invites mistakes. Backward compatibility is what makes consolidation possible:

1. List every engine and its manual specification — viscosity grade and API category or OEM standard.
2. Find the highest common category. If the newest engines need CK-4 15W-40 and the older ones need CH-4 or CI-4 15W-40, a CK-4 15W-40 can normally cover all of them.
3. Flag the exceptions. Engines that need a different grade (such as 20W-40 or a 30 grade), an FA-4 oil or a specific OEM standard stay on their own oil.
4. Label the store. Mark drums and dispensing pumps clearly so the low-SAPS oil goes into the after-treatment engines and nothing older gets topped up with the wrong grade.

The cost question is real: an older CH-4 engine does not need CK-4, so a fleet with no BS-VI vehicles may reasonably stay on CI-4. But once a single DPF-equipped engine arrives, consolidating on the newer category usually costs less than managing two diesel oils and the risk of a mix-up.

Which rating do Indian engines need?

For most trucks, tractors and gensets in India, API CH-4 or CI-4 covers the requirement, and CI-4 is a strong, widely-specified choice. Newer BS-VI and heavy-duty highway engines may call for CK-4. The safe approach is simple: read the manual, match both the viscosity and the API rating, and buy from a brand whose rating you can trust.

How the rating shows up in service: soot, base number and oil analysis

The real-world difference between categories is mostly about how long the oil keeps working under soot, heat and acid. You can see this in the engine without guessing.

• Soot handling. EGR engines push more soot into the oil. A good oil keeps it finely dispersed; an overloaded one lets it clump, thickening the oil and wearing the valve train. The blotter spot test shows this: put a drop of warm oil on white filter paper and leave it a few hours. An even grey spread is healthy; a dense black centre with a hard edge means the dispersant is exhausted.
• Base number. The oil’s alkaline reserve (TBN) neutralises acids from combustion. It falls steadily in service, and when it drops too low, corrosive wear rises.
• Viscosity drift. Rising viscosity points to soot or oxidation; falling viscosity points to fuel dilution or shear. Newer categories set tighter limits on both.

For fleets, a periodic lab report should cover viscosity at 100 °C, soot, TBN, fuel dilution, water, silicon (dust ingress, common on Indian sites and haul roads) and wear metals such as iron, copper and lead. It is the only reliable way to judge whether a longer drain is safe. Without it, keep to the interval in the manual and use its severe-service interval for dusty, overloaded, idling-heavy or short-trip work. A better API category gives more margin; it does not replace the manual.

When the wrong category is in the engine

A category mismatch rarely causes a sudden failure; it shows up slowly.

For engines under warranty, keep the invoice, pack category and batch number with the service record as proof the specified oil was used.

Decision checklist before you buy

1. Read the manual. Note the viscosity grade and the API category or OEM standard for each engine.
2. Check for after-treatment. A DPF or catalyst in the exhaust means low-SAPS — CJ-4, CK-4 or what the maker names.
3. Rule out FA-4 confusion. Use FA-4 only where the manual calls for it; everywhere else, CK-4 is the backward-compatible choice.
4. Match the grade separately. Rating and viscosity are two decisions. See 15W-40 vs 20W-40 for the grade side.
5. Ask for the data sheet. The API category should be printed on both the pack and the product data sheet.
6. Set drains by duty. Use the manual’s interval, shorten it for severe service, and extend only with oil analysis.

Haruto NexaGuard engine oils come in 15W-40 and 20W-40 for diesel trucks, tractors, earthmoving equipment, gensets and pump sets, in packs up to 50 L and 210 L drums for fleet stores. Request the data sheet for your grade and match its category to your manual. For fleet-wide advice, read our guide to the best engine oil for trucks and buses in India.