A viscosity index improver (VII) is the additive that makes multigrade oils possible. It thickens oil at high temperatures to maintain the hot viscosity grade, while keeping the oil thin enough at low temperatures for easy cold starts. Without VIIs, 10W-40, 5W-30 and 0W-20 oils would not exist. This guide compares the main VII chemistries and how to choose.
How a Viscosity Index Improver Works
VII polymers are long-chain molecules that are soluble in base oil. At low temperature they coil up and contribute little viscosity; as temperature rises they expand and increase viscosity, counteracting the natural thinning of the base oil. The result is a flatter viscosity-temperature curve – a higher viscosity index – and stable viscosity across the operating range.
The Main VII Chemistries
| Type | Minglan grades | Key property | Typical use |
|---|---|---|---|
| OCP (olefin copolymer) | J0010/J0050, OCP-010/030/050, T612/T614/T616 | Excellent thickening and thermal stability; SSI 15-45 | Engine oils, hydraulic oils |
| PMA (polymethacrylate) | V6805, T819HB, T820, T823 | Very high shear stability (low SSI), also improves pour point | ATF, gear oils, high-shear applications |
| PIB (polyisobutylene) | PIB950/1300/2400 | Low cost, good thickening at low shear | Gear oils, two-stroke oils |
| Styrenic (HSI) | V126 | Very high shear stability, SSI 12 | Top-tier multigrade engine oils |
Shear Stability: The Hidden Spec
The most important VII property is shear stability, measured as SSI (shear stability index). A VII with high SSI loses thickening power permanently as the polymer chains are cut in service, causing the oil to drop a viscosity grade. Modern high-performance engine oils require low-SSI VIIs such as V126 (SSI 12) or PMA grades, while industrial oils can accept lower-cost OCP types.
Selection Checklist
- Define the target SAE grade pair (e.g. 10W-40) and the base oil viscosity.
- Choose the chemistry by shear stability requirement and cost.
- Verify thickening power in your base oil with the actual treat rate (typically 3-10%).
- Check compatibility with PPD and the additive package – PMA VIIs often act as PPDs too.
Source VII From a Full-Line Manufacturer
Shanghai Minglan Chemical supplies the complete VII portfolio – OCP, PMA, styrenic and PIB – enabling formulators to source all viscosity modifiers from one manufacturer. Contact info@minglanchem.com for specifications and samples.
