Heavy-duty diesel engine oils explained — CH-4, CI-4 & CK-4.

"15W-40 CH-4." It's printed on millions of drums across India and quoted in every fleet specification — and most people who read it can't explain what the CH-4 actually means. Here's the field guide we wish every fleet operator had.

What the API "C" categories actually rate

The American Petroleum Institute classifies heavy-duty diesel engine oils with a two-letter prefix: C for compression-ignition (diesel) engines, followed by a sequence letter (D, F, G, H, I, J, K). Higher letters indicate newer-generation oils designed for tighter emission norms and improved engine technology. Every newer category is backward-compatible with older ones — you can use CK-4 in an engine that calls for CI-4 or CH-4.

API Category	Year Introduced	Engine Generation	Status
API CF-4	1990	Pre-emission diesel	Obsolete
API CG-4	1995	Early emission-control	Obsolete
API CH-4	1998	Post-1998 emission norms	Active in India
API CI-4	2002	Cooled EGR engines	Active
API CJ-4	2006	DPF-equipped engines, low SAPS	Active
API CK-4	2017	SCR + DPF, BS-VI / Euro VI	Current
API FA-4	2017	Fuel-economy spec, thinner viscosity	Niche (not BC)

API CH-4 — the Indian workshop standard

For most pre-BS-IV diesel vehicles still on Indian roads — older Tata, Ashok Leyland, Eicher trucks, tractors, gensets, construction equipment — API CH-4 15W-40 remains the workshop default. It provides:

• Strong wear protection at high soot loads
• Stable viscosity under sustained high temperatures
• Detergent/dispersant additives sized for older injector designs
• Typical drain interval: 20,000–25,000 km in normal duty

It's still cheap, widely available, and entirely appropriate for engines designed around it. The mistake is using it in a BS-VI engine with a DPF — the higher ash content will clog the filter over time.

API CI-4 — for cooled-EGR engines

CI-4 was designed for engines that introduced cooled exhaust gas recirculation around 2002. Compared to CH-4 it offers:

• Better soot-handling (EGR engines see more soot in the oil)
• Improved oxidation stability
• Better wear protection on valve trains and rings
• Drain intervals typically 30,000–40,000 km

Fleets running mid-generation BS-III and BS-IV vehicles benefit from stepping up to CI-4 — slightly higher per-litre cost, longer drain, less wear.

API CK-4 — current generation, BS-VI ready

CK-4 was introduced in 2017 and is the current top tier for heavy-duty diesel. It's backward-compatible with CJ-4, CI-4, CH-4, and CF-4 — meaning a single CK-4 oil can service an entire mixed-age fleet. CK-4 brings:

• Lower SAPS (Sulfated Ash, Phosphorus, Sulfur) — protects DPF and SCR aftertreatment
• Better oxidation stability for longer drains
• Improved shear stability — viscosity holds across the drain
• Better aeration control under high-RPM conditions
• Drain intervals up to 40,000–60,000 km in modern fleets

If you're running BS-VI Tata, Ashok Leyland, Volvo, or Scania, CK-4 is the right choice. Trying to save money with CH-4 in a DPF-equipped engine is false economy — DPF replacement costs more than years of premium oil.

SAE viscosity — pairing the right grade

API tells you the additive quality. SAE tells you the viscosity. Common diesel viscosities in India:

• SAE 15W-40 — the workhorse. All-season for most of India, suits CH-4, CI-4 and CK-4 oils. The default for trucks, buses, tractors, gensets.
• SAE 20W-40 / 20W-50 — older engines, hot regions, high-load monsoon conditions.
• SAE 10W-30 — modern BS-VI engines designed for fuel-economy oils, cold-region winter operation.
• SAE 5W-30 / 5W-40 — premium synthetic, Himalayan / sub-zero applications.

The cost-vs-life math

A litre of CK-4 costs more than CH-4. But a 60,000 km drain interval at CK-4 beats two 25,000 km drains at CH-4 — including the cost of the lost two days of fleet downtime for the extra service.

For modern fleets, total cost of ownership almost always favours the highest API category your engine warranty allows. For older vehicles, CH-4 remains the rational choice.

How to choose for your fleet

1. Check the OEM spec. Modern owner's manuals list the exact API category and SAE viscosity. Follow it.
2. Match the engine generation. Pre-BS-III → CH-4. BS-III/IV → CI-4 (or CK-4 if budget allows). BS-VI → CK-4.
3. Consider duty cycle. Long-haul highway = use the OEM drain. Stop-go, idling, dust = drain at 60–70% of OEM interval.
4. Don't downgrade. A newer engine on older oil voids warranty and damages aftertreatment.
5. Buy from a manufacturer, not a re-bottler. Ask for the Certificate of Analysis with each batch.

What we manufacture

Manak Petroleum has been blending heavy-duty diesel engine oils in India for fifty years. Current production includes:

• ZIGMA CH-4 15W-40 — heavy-duty diesel, the fleet standard, available in 10 L pack
• EMTEX TC Qwik CI-4+ 15W-40 — premium CI-4 plus, longer drain
• EMTEX Xpresso 20W-40 — high-load high-temperature commercial vehicle oil
• EMTEX Chota Hathi 20W-40 — small commercial vehicle and tractor segment

See the full spec sheets on the engine oils catalogue, or launch your own private-label heavy-duty diesel engine oil from our ISO 9001:2015 facility with batch documentation and pan-India dispatch.