Viscosity index improvers (VII / VM = viscosity modifier) are the polymer additives that make multigrade oil possible — they thicken oil at high temperature without thickening it as much at low temperature, giving the SAE 5W-30, 15W-40 multigrade characteristic. OCP (olefin copolymer) is the workhorse; PMA (polymethacrylate) is the multifunctional premium chemistry; PIB (polyisobutylene) is for gear oil; star polymer is racing-grade specialty. This guide covers chemistry families, shear stability index (SSI), treat rate by service and Indian sourcing.
| VII Type | SSI Range | Treat % (finished) | VI Lift | Best Service |
|---|---|---|---|---|
| OCP solid SSI-25 | 22-28 | 6-9% | 80-95 | PCMO 5W-30 / 5W-40 API SP |
| OCP solid SSI-35 | 30-38 | 7-10% | 80-100 | HDD 15W-40 / 10W-30 CK-4 |
| OCP solid SSI-45 | 40-48 | 9-12% | 75-95 | HDD 20W-50, lower-tier multigrade |
| PMA SSI-20 | 18-25 | 3-6% | 60-85 | GF-6 0W-20 / 5W-30 premium |
| Star polymer SSI-18 | 15-22 | 2-4% | 55-80 | Racing, premium 0W-20 |
| DD-PMA hybrid | 25-32 | 4-7% | 70-90 | HDD CK-4 / CI-4+ extended drain |
| SAE Grade | Base Oil | VII Type | Treat % (finished) |
|---|---|---|---|
| SAE 0W-20 | Group III 4cSt + PAO 4cSt | PMA SSI-20 or star | 2.5-4.5% |
| SAE 5W-30 | Group II 110N + Group III 4cSt | OCP SSI-25 or PMA SSI-22 | 5-8% |
| SAE 5W-40 | Group II 110N + 220N | OCP SSI-30 | 8-11% |
| SAE 10W-30 | Group I SN150 + SN500 | OCP SSI-30 | 6-8% |
| SAE 10W-40 | Group I SN150 + SN500 | OCP SSI-30 | 8-11% |
| SAE 15W-40 HDD | Group I SN150 + SN500 | OCP SSI-35 | 7-10% |
| SAE 75W-90 Gear GL-5 | Group II 220N + 600N + bright | PIB Mn 1300-2300 | 8-14% |
| ATF Dexron VI | Group III 4cSt | PMA SSI-15 | 7-10% |
Neither universally — they serve different roles. OCP (olefin copolymer, ethylene-propylene) is the lowest-cost VII per VI lift, workhorse for PCMO and HDD multigrade. Solid OCP gives high-SSI versions (shear stable) and liquid OCP gives low-SSI versions.
PMA (polymethacrylate copolymer) is 30-50% more expensive but multifunctional — built-in pour point depression (no separate PPD needed), better low-temp viscometrics, better fuel-economy retention (Mo-DTC + PMA synergy). PMA is the standard for ILSAC GF-6 / GF-7 premium PCMO. PIB is used in gear oil and some hydraulic, not multigrade engine oil. Star polymer (hydrogenated styrene-isoprene) is racing-grade specialty.
SSI = Shear Stability Index, measured by ASTM D6278 (Bosch injector shear) or KRL tapered roller bearing shear. SSI is the % viscosity loss after a defined shear cycle — lower SSI = better shear stability = polymer chains hold up under shear.
PCMO API SP / GF-6 typically uses SSI 25-35 OCP. HDD CK-4 uses SSI 35-50 OCP. Hydraulic and racing oil use SSI 15-25 (very shear-stable). High SSI VII (>50) loses too much viscosity in service — KV100 drops below SAE J300 grade minimum after 100 hr engine operation.
Treat rate depends on base oil viscosity and target finished-oil viscosity. For SAE 5W-30 from Group II 110N base, VII treat is 6-8% OCP SSI-30. For SAE 5W-40 from same base, treat is 9-11%. For SAE 15W-40 from Group I SN150 + SN500 mix, treat is 4-6%.
PMA at half the treat rate of OCP gives equivalent VI lift but at 2-3x the cost — net cost slightly higher but lower MRV / CCS gives better cold-start. The blender calculates treat by KV100 target and VII KV100-thickening efficiency at SSI level.
Two failure modes. Temporary VI loss: at high shear (engine bearing, gear mesh, hydraulic pump) the polymer coil temporarily aligns with the flow direction, lowering its viscosity contribution — recovers when shear stops. Measured by HTHS (high-temperature high-shear viscosity, ASTM D4683).
Permanent VI loss: polymer chain physically breaks under high shear, reducing molecular weight permanently. Measured by KRL or D6278 SSI. Both matter — HTHS dictates fuel economy and engine protection at bearing; SSI dictates oil-change-interval viscosity retention.
Star polymer (hydrogenated styrene-isoprene block copolymer in star architecture) has higher viscosity-thickening efficiency per molecular weight than OCP, giving equivalent VI lift at lower treat rate. Plus very low SSI (15-25).
Drawback: cost is 3-4x OCP. Used in racing oil, premium PCMO ILSAC GF-6 0W-20, and high-shear hydraulic. Indian volume is small but premium-tier OEM aftermarket increasingly uses star polymer (or star-OCP hybrid like Lubrizol Viscobase HSD).
OCP solid (Lubrizol Viscobase 9, Afton HiTEC 5777, Infineum V387, Mitsui Lucant) lands at ₹240-340 / kg. OCP liquid concentrate at ₹180-260 / kg. PMA (Evonik Viscoplex 5151, Lubrizol Sandura) at ₹420-560 / kg. Star polymer at ₹650-950 / kg.
Lubechem custom OCP equivalent at ₹160-230 / kg (30-32% saving on branded). PIB Mn 950-2300 from BASF Glissopal, INEOS Indopol at ₹200-320 / kg.
DD-PMA (dispersant-modified polymethacrylate) is PMA copolymerised with dispersant-functional comonomers (e.g. N-vinylpyrrolidone, dimethylaminoethyl methacrylate). Combines VII function with dispersancy contribution — reduces the need for separate succinimide dispersant.
Used in HDD CK-4 and CI-4+ for extended drain. Treat 4-7% in finished. Cost is 1.5-2x conventional OCP but saves on separate dispersant treat. Brands: Evonik Viscoplex 10-series, Lubrizol Viscobase DD, Afton HiTEC 3419.
For mid-volume blenders (>500 MT/year multigrade), custom OCP equivalent saves ₹20-40 lakh per year. The OCP route (ethylene-propylene Ziegler-Natta polymerisation, MW control by hydrogen, dissolution in 100N base oil) is well-established.
Indian blenders typically buy custom-blended VII rather than manufacturing in-house — Indian polymerisation reactor capacity for OCP is limited; PMA can be made at smaller scale. PMA custom is more accessible at small MOQ (200-500 kg). OCP custom usually requires 5 MT minimum batch.
Share your target SAE grade, base-oil type, SSI requirement and volume. We respond within one business day with a VII selection plan, treat rate calculation and indicative cost.