15W-40 is the workhorse heavy-duty diesel engine oil for the Indian commercial vehicle, off-highway, generator, and agricultural diesel market. This guide walks through the realistic composition of a 15W-40 formulation — base oil selection, viscosity index improver, detergent-inhibitor package, antiwear chemistry, pour point depressant, and defoamer — with treat-rate ranges, API category differences, BIS IS 13656 pass criteria, and the four-step manufacturing process.
The “15W-40” designation comes from SAE J300, the Society of Automotive Engineers viscosity classification for engine oils. It is a multigrade oil — meaning it meets viscosity requirements at both low and high operating temperatures.
15W is the “winter” rating — it commits the oil to a specific maximum viscosity at low temperature, ensuring acceptable cold-start performance. Specifically, a 15W oil must pump at −25 °C (ASTM D4684 MRV limit 60,000 cP max) and crank at −20 °C (ASTM D5293 CCS limit 7,000 cP max).
40 is the high-temperature rating — it commits the oil to a kinematic viscosity at 100 °C of 12.5 to 16.3 cSt (ASTM D445) and an HTHS viscosity of 3.7 cP min at 150 °C (ASTM D5481). This range ensures the oil maintains an adequate hydrodynamic film under engine bearing loads at operating temperature.
A monograde oil (e.g. SAE 40) meets only the high-temperature requirement. A multigrade 15W-40 meets both — achieved by adding a viscosity index improver (VII) polymer to the base oil so it thins less as temperature rises. This is why VII is the single largest non-base-oil component in a 15W-40 formula.
| Component | Function | Typical % (m/m) | Notes |
|---|---|---|---|
| Base oil — Group I SN500 | Bulk viscosity backbone; mid-viscosity component | 55–65% | Workhorse for HD-2 / CH-4 grades |
| Base oil — Group I SN150 | Light viscosity component; trim for low-temp performance | 15–25% | Trim ratio adjusted to hit CCS limit |
| VII — OCP polymer (10–12% active) | Viscosity index improvement; multigrade behaviour | 7–9% | OCP is industry standard; HSD for premium |
| Overbased calcium sulfonate | Detergent — piston deposit control, TBN reserve | 2.5–4% | Higher % for CI-4 / CK-4 categories |
| Calcium phenate (optional) | Antioxidant / detergent boost — high-temperature deposit control | 0.8–1.5% | Common in CI-4 Plus and CK-4 |
| PIBSI dispersant | Soot dispersion, sludge control, neutralisation of acidic combustion products | 3–5% | Critical for diesel soot handling; raised for CI-4+ |
| ZDDP (primary / secondary alkyl) | Antiwear and antioxidant — protects camshafts, cam followers, valvetrain | 0.9–1.4% | Treat rate gives 0.08–0.12% P in finish |
| Alkylated diphenylamine (ADPA) | Antioxidant — extends oil life, controls viscosity rise | 0.3–0.8% | Higher in low-SAPS CK-4 to compensate for reduced ZDDP |
| Pour point depressant (PPD) | Lowers pour point of base oil blend | 0.1–0.3% | PMA-type; treat rate depends on base oil source |
| Silicone defoamer | Suppresses foaming during operation | 5–20 ppm | Diluted to dosable concentration before addition |
| Corrosion inhibitor (optional) | Yellow-metal corrosion protection | 0.05–0.15% | Often integrated into DI pack |
The above represents a component-blending approach. When using a commercial DI package (Lubrizol, Infineum, Chevron Oronite, Afton), the detergent, dispersant, ZDDP, antioxidant, and corrosion inhibitor are pre-blended into a single concentrate dosed at 9 to 13% in finished oil — reducing handling complexity at the cost of slightly higher unit price.
| Property | API CH-4 | API CI-4 | API CI-4 Plus | API CK-4 |
|---|---|---|---|---|
| Target engines | Pre-BS-IV diesel | BS-III / BS-IV diesel | BS-IV diesel with EGR | BS-VI / SCR equipped |
| TBN target (ASTM D2896) | 9–11 mgKOH/g | 10–12 mgKOH/g | 10–12 mgKOH/g | 8–10 mgKOH/g |
| Soot handling capacity | 4% max soot | 6% max soot | 7% max soot | 7%+ max soot |
| Sulphated ash (D874) | 1.4–1.7% | 1.4–1.7% | 1.4–1.7% | 1.0% max (low-SAPS) |
| Phosphorus (D5185) | 0.10–0.14% | 0.10–0.14% | 0.10–0.14% | 0.12% max |
| Sulphur | No limit | No limit | No limit | 0.4% max |
| Oxidation stability | Standard | Enhanced | Enhanced | Significantly enhanced |
| Aftertreatment compatibility | Not required | Not required | Compatible | DPF / SCR compatible |
| Typical DI pack treat rate | 9.5–10.5% | 10.5–12% | 11–12.5% | 11–13% |
| Indicative cost premium vs CH-4 | Baseline | +Rs 5–9 / L | +Rs 10–18 / L | +Rs 18–28 / L |
The key shift between CI-4 Plus and CK-4 is the introduction of low-SAPS (low Sulphated Ash, Phosphorus, Sulphur) chemistry. CK-4 is designed for engines with diesel particulate filters and SCR systems — high SAPS oils poison the catalysts. CK-4 reformulation is the single most material chemistry transition happening in heavy-duty engine oil right now in India.
| Property | ASTM Method | IS 13656 HD-3 Limit | Recommended Formulation Target |
|---|---|---|---|
| KV @ 100 °C | D445 | 12.5–16.3 cSt | 14.0–14.8 cSt |
| KV @ 40 °C | D445 | Report | 100–115 cSt |
| Viscosity Index | D2270 | 125 min | 138–145 |
| CCS @ −20 °C | D5293 | 7,000 cP max | 5,500–6,200 cP |
| Pour point | D97 | −15 °C max | −24 to −30 °C |
| Flash point (COC) | D92 | 200 °C min | 220–240 °C |
| TBN | D2896 | 7 mgKOH/g min | 10–12 mgKOH/g |
| Sulphated ash | D874 | 1.5% max | 1.35–1.45% |
| Foam Seq I / II / III | D892 | 10/0, 50/0, 10/0 ml max | Nil / nil / nil |
| Four-ball wear scar | D4172 | 0.6 mm max | 0.45–0.52 mm |
| NOACK volatility | D5800 | 15% max | 9–12% |
The “Recommended Formulation Target” column is the design target we work to in our 15W-40 formulations. The intent is to comfortably clear the IS 13656 limit on every property so that production batch variation never threatens compliance.
Yes — Group I SN500 / SN150 blend remains the workhorse for 15W-40 in India. Group II is increasingly preferred for low-SAPS CI-4 Plus and CK-4 grades because of better oxidation stability and lower NOACK volatility, but Group I is still the dominant base for HD-2 and HD-3 commercial formulations. For CK-4, Group II is strongly recommended because the constrained additive headroom puts more performance burden on the base oil.
ZDDP is added at 0.9 to 1.4% to deliver 0.08 to 0.12% phosphorus in finished oil for CH-4 and CI-4 categories. For low-SAPS CI-4 Plus and CK-4 the P limit drops to 0.12% max (and often a 0.08% target is preferred to give DPF protection margin), so ZDDP treat rate is reduced and the antiwear performance is balanced with ash-free chemistry — typically additional ADPA antioxidant and selective dispersant boost.
CK-4 targets a TBN of 8 to 10 mgKOH/g (ASTM D2896) — high enough for acid neutralisation through the drain interval but constrained by the SAPS limits (1.0% sulphated ash max). CI-4 typically runs 10 to 12 TBN. CH-4 runs 9 to 11. The CK-4 reduction in TBN is offset by significantly more robust oxidation chemistry and improved soot dispersancy.
Direct material cost for a CH-4 15W-40 ranges from Rs 110 to 130 per litre at current base oil and additive prices. CI-4 Plus adds Rs 10 to 18 per litre due to richer DI pack. CK-4 adds Rs 18 to 28 per litre because of low-SAPS DI chemistry, higher antioxidant content, and the preference for Group II base oil. Add Rs 6 to 10 per litre for packaging, labour, utilities, and overhead.
15W-40 has lower low-temperature viscosity (better cold start — pumps at −25 °C versus −20 °C for 20W) and slightly thinner high-temperature viscosity (KV100 12.5–16.3 cSt versus 16.3–21.9 cSt for 50-grade). 15W-40 is the workhorse for heavy-duty diesel and modern petrol; 20W-50 is preferred for older engines and high-ambient applications where wear protection at temperature is the priority. Two-wheeler and tropical climate users tend toward 20W-50; truck and modern OEM-spec users tend toward 15W-40.
Commercial DI packs from Lubrizol, Infineum, Chevron Oronite, and Afton are reliable, give predictable performance, and shortcut the formulation timeline — the right answer for most new entrants. Custom component blending (individual detergent, dispersant, ZDDP, antioxidant) can save 8 to 14% on additive cost at scale but needs strong formulation control, a robust QC lab, and ongoing supplier qualification. Component blending makes sense above 500 KL/month of a given grade family — below that, commercial DI packs win on total cost of ownership.
Share your target API category (CH-4 / CI-4 / CI-4 Plus / CK-4), production scale, and price band. We’ll respond within one business day with an indicative formulation plan, BOM, and BIS certification roadmap.