- A viscosity index improver (VII) is a polymer that reduces how much an oil's viscosity changes with temperature — what makes multigrades such as 5W-30 and 10W-40 possible.
- Viscosity index is a dimensionless number (ASTM D2270) from KV at 40°C and 100°C: mineral oils ~95–105, PAO 140–150, finished oils with a VII 160–200+.
- VIIs work by coil-and-stretch — coiled and low-impact when cold, expanded and thickening when hot.
- Judge a VII on thickening efficiency (TE), shear-stability index (SSI, ASTM D6278) and HTHS contribution.
- Four chemistries: OCP (low-cost mainstream), PMA (low-temp + pour-point depressant), HSD (high-TE premium) and PIB (thickener / dispersant-VII base).
- Mechanical shear permanently breaks chains, so specs set post-shear KV100 minimums (API SP requires ≥9.3 cSt for a 5W-30).
A viscosity index improver (VII) is a polymer additive that reduces the rate at which an oil’s viscosity changes with temperature. Without a VII, a mineral base oil that flows well at 100°C would be too thick to pump at -20°C. VIIs are what make modern multi-grade oils — 5W-30, 10W-40, 15W-40 — possible.
What Is Viscosity Index?
Viscosity index (VI) is a dimensionless number that describes how much an oil’s kinematic viscosity changes between 40°C and 100°C. A higher VI means less viscosity change — the oil stays more consistent across temperature extremes. Pure mineral base oils typically have VI values of 95–105. Synthetic PAO base oils reach 140–150. With a good VII at typical treat rates, finished oils can reach VI values of 160–200+.
VI is calculated from kinematic viscosity at 40°C and 100°C using ASTM D2270. It is not measured directly.
How Viscosity Index Improvers Work
VIIs are long-chain polymers with coil-and-stretch behaviour. At low temperatures, the polymer chains contract into tight coils that have minimal impact on oil viscosity — the oil flows as if the polymer were not there. At high temperatures, the chains expand and entangle, thickening the oil. This temperature-dependent thickening partially compensates for the natural thinning of the base oil at elevated temperatures.
The key metrics for a VII are:
- Thickening efficiency (TE): how much kinematic viscosity increase the polymer produces per unit of polymer in solution
- Shear stability index (SSI): the percentage viscosity loss after mechanical shearing (ASTM D6278, CEC L-45). A lower SSI means a more shear-stable polymer
- HTHS viscosity contribution: the polymer’s effect on high-temperature high-shear (HTHS) viscosity at 150°C/10⁶ s⁻¹, the closest simulation of bearing film thickness under peak engine load
Main VII Polymer Types
There are four major commercial VII chemistries:
| VII chemistry | Shear stability & cost | Typical use |
|---|---|---|
| OCP (Olefin Copolymer) | High TE, moderate SSI (15–35%), low cost | Nearly all PCMO and HDDO multi-grade oils |
| PMA (Polymethacrylate) | Good shear stability; also a pour-point depressant; higher cost than OCP | ATF, gear oils, energy-conserving engine oils; excellent low-temperature performance |
| HSD (Hydrogenated Styrene Diene) | Very high TE, good shear stability; star-shaped polymer | Premium PCMO and fuel-economy oils |
| PIB (Polyisobutylene) | Low VI improvement | Base for dispersant-VII multifunctionals; grease thickener |
CheMost supplies viscosity index improvers including OCP and PMA grades for engine oil, gear oil, and ATF applications, with SSI values from 10–45% depending on the grade.
From the labFormulating motor oil or transmission fluid? CheMost supplies the chemistry.View engine oil packagesShear Stability and Viscosity Retention
Under engine conditions, high mechanical shear (especially in the oil pump, journal bearings, and valve train) can permanently break polymer chains. This lowers the oil’s viscosity grade over time — a process called viscosity loss on shear. An oil formulated to meet 5W-30 at fill may drop to the equivalent of a 5W-20 in service if the VII shears excessively.
This is why modern engine oil specifications set minimum KV100 limits not just for fresh oil but also after standardised shear tests. For example, API SP specifies a minimum KV100 of 9.3 cSt after shear for a 5W-30 oil.
For heavy-duty applications, ACEA E6/E9 requires the oil to stay within its viscosity grade after 250+ hours of operation — necessitating high-stability OCP or PMA grades.
Frequently Asked Questions
What is the difference between KV40, KV100, and HTHS viscosity?
KV40 and KV100 are kinematic viscosities (in cSt) measured at 40°C and 100°C respectively, under no external shear force (ASTM D445). HTHS is high-temperature high-shear viscosity measured at 150°C and 10⁶ s⁻¹ (ASTM D4683), which simulates conditions in engine bearings. HTHS is a better predictor of engine wear than KV100 alone and is increasingly used as a specification target.
Do VIIs cause sludge or varnish?
OCP-based VIIs can contribute to low-temperature sludge if the ethylene content is too high, because high-ethylene OCP can crystallise at low temperatures. Modern OCPs are designed with 50–60% ethylene content to balance VI improvement and low-temperature performance. PMA VIIs are inherently more resistant to sludge issues.
Can I add a VII directly to base oil to make a multi-grade?
Technically yes, but a finished lubricant requires a full additive package (antiwear, detergent, dispersant, antioxidant, etc.). Adding a VII alone to base oil gives a thicker, more thermally stable oil — but without the performance chemistry required by engine oil specifications. VII is always one component of a balanced formulation.
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