By Technical Team, CheMost Additives    |  12 min read    |  Last updated: 2025-11-11

How to Improve Viscosity Index

TL;DR — Who This Is For & What You'll Get
 
For formulators and blenders who need to raise the viscosity index of a finished lubricant — either to hit a multi-grade specification or to improve low-temperature performance without sacrificing high-temperature protection. You'll learn the two methods (VII polymers and synthetic base oils), the trade-offs of each, and the hybrid approach that most commercial formulations actually use.

Key Takeaways

• VI measures how much viscosity changes with temperature — higher VI means the oil stays more consistent from cold start to operating temperature
• Two methods exist: add a VII polymer (cheaper, treat rate 3–12%) or switch to a higher-VI synthetic base oil (better shear stability, no polymer degradation)
• VII polymers work by expanding at high temperature (thickening the oil when hot) and contracting at low temperature (minimizing cold drag) — the coil-expansion mechanism
• Synthetic base oils (Group III, PAO) have naturally higher VI (130–150+) without polymers — but cost 2–5× mineral oil and have lower additive solubility
• Most commercial formulations use both: a Group II/III base oil with a VII polymer at 3–8% treat rate — the cost-effective middle ground

Viscosity index — VI — is a number that tells you how much an oil's viscosity changes with temperature. A VI of 100 means moderate change. Below 80, the oil thins aggressively with heat and thickens like honey in the cold. Above 140, it barely changes — roughly the same fluidity at -20°C as at 100°C.

The number matters because engines don't run at one temperature. An oil that's thick enough to protect bearings at 150°C must still flow well enough to crank the engine at -25°C. VI is what bridges that gap. And there are exactly two ways to improve it: add a viscosity index improver polymer, or start with a higher-VI base oil. Everything else is a variation on one of these two.

CheMost manufactures VII polymers — OCP, PMA, and HSD — and provides base oil evaluation support for blenders optimizing VI across their product lines.

Method 1: Add a Viscosity Index Improver Polymer

A viscosity index improver is a polymer that changes shape with temperature. At low temperature, the polymer chain coils tightly — it occupies minimal hydrodynamic volume and contributes little to viscosity. This is what you want during a cold start: the polymer stays out of the way, and the base oil's natural cold viscosity (plus any pour point depressant) determines cranking resistance. At high temperature, the chain uncoils — expanding its hydrodynamic volume and thickening the oil precisely when you need the film strength.

This coil-expansion mechanism is why VII works at all. A polymer that doesn't change shape with temperature would just add a constant viscosity offset — raising both cold and hot viscosity equally, which defeats the purpose.

How much VI improvement can you get from a VII? Depends on the polymer, the treat rate, and the starting base oil:

The pattern: PMA delivers more VI improvement per unit of treat rate. OCP requires more polymer but costs less per kilogram. The formulator's math is VI-gain-per-dollar, not VI-gain-per-kilogram.

There's a catch. The VII polymer adds VI at the cost of shear stability. Every VII has an SSI (Shear Stability Index) — the percentage of its thickening contribution lost during mechanical shear. A polymer with SSI 30 loses 30% of its thickening effect after the standard 30-cycle Kurt Orbahn test. This means the VI you measured in the lab on fresh oil will be lower after 5,000 km in service. Formulators account for this by targeting a fresh-oil viscosity above the minimum — knowing the oil will shear down to a stable viscosity within the first few thousand kilometers.

Method 2: Switch to a Synthetic Base Oil

The second method doesn't add anything. It changes what you start with.

Synthetic base oils — Group III (severely hydrocracked mineral), Group IV (PAO), and Group V (esters) — have naturally higher VI than Group I/II mineral oils. No polymer needed. The VI comes from the molecular structure: narrow molecular weight distribution, minimal wax content, and in PAO's case, a controlled oligomer structure with a built-in viscosity-temperature profile:

The advantage of the synthetic route is permanence. A PAO-based oil with VI 145 will have VI 145 at 0 km, at 10,000 km, and at 50,000 km — no polymer to shear down, no viscosity loss over time. The VI is in the molecules, not in a polymer that mechanical stress can destroy.

The disadvantage: cost and additive solubility. PAO has poor additive solubility — polar additives like ZDDP, sulfonate detergents, and PIBSI dispersants can precipitate or form haze in PAO without a co-solvent. The standard fix is 5–15% diester added to the PAO base. The ester solubilizes the additives while the PAO provides the VI and oxidation stability. This is what "fully synthetic" engine oil usually means — a PAO/ester blend, not pure PAO.

The Hybrid Approach: What Most Formulations Actually Do

In practice, almost no commercial engine oil is purely polymer-thickened mineral or purely synthetic. The standard approach is a hybrid:

For a 5W-30 passenger car oil, the semi-synthetic route — Group II/III blend with 5% PMA — hits the CCS, MRV, and HTHS targets at roughly 40% of the cost of a full PAO formulation. The polymer provides the last 20–30 VI points. The synthetic portion of the base oil provides the first 20–30 VI points. Neither alone would be cost-effective; together, they're the industry standard.

A word of caution. Dropping a high-VI synthetic base oil into an existing formulation and expecting the finished oil to pass all specs doesn't work. The additive package was designed for a specific base oil solvency and response. Switch to a lower-solvency base (Group III, PAO), and the detergent may not stay dispersed, the anti-wear additive may not activate at the same rate, and the demulsibility behavior can shift. Formulate the package for the base oil — don't swap bases under an existing package.

Optimizing VI for your formulation? Tell us your target SAE grade, base oil type, and cost constraints → — we'll recommend a VII polymer with SSI and treat-rate data.

Frequently Asked Questions

Related Articles

• What are Viscosity Index Improvers? — Full VII overview: OCP, PMA, PIB, HSD chemistry and selection.
• OCP Viscosity Index Improver — OCP VII: cost-effective shear stability for HDDO and industrial oils.
• PMA Viscosity Index Improver — PMA VII: best low-temperature performance and dual VII+PPD function.
• HSD Viscosity Index Improver — HSD VII: star architecture for balanced performance.
• Thickeners & Viscosity Index Improvers — CheMost's full VII product line.

References & Industry Standards

• ASTM International: ASTM D2270 — Calculating Viscosity Index from Kinematic Viscosity
• ASTM International: ASTM D6278 — Shear Stability of Polymer-Containing Fluids
• SAE International: SAE J300 — Engine Oil Viscosity Classification