⛽ Viscosity index (VI) of oils

Petrochemistry PN-EN ISO 2909

Overview

Viscosity Index (VI) is a dimensionless number defining the change in oil viscosity as a function of temperature. The higher the VI, the smaller the viscosity change with temperature change — the oil maintains more stable lubricating properties at both low and high temperatures. This parameter has fundamental significance in selection of lubricating oils for applications operating over a wide temperature range.

The VI calculation method was developed by Dean and Davis in 1929 and is based on comparing the test oil viscosity with viscosities of two reference oil series — with low (VI = 0, L series) and high (VI = 100, H series) viscosity index — having the same viscosity at 100°C. ISO 2909 standard defines two procedures: A (for VI ≤ 100) and B (for VI > 100), with different calculation formulas.

Typical VI values: naphthenic oils (group I) — 80–100, refined paraffinic oils (group II) — 95–110, hydrocracked oils (group III) — 120–140, PAO (group IV) — 130–160, synthetic esters (group V) — 130–170. Modern multigrade engine oils (e.g., 5W-30) achieve VI > 150 thanks to polymeric viscosity index improvers (VII — Viscosity Index Improvers).

Viscosity index is a calculated parameter — it does not require a separate test, but results from kinematic viscosity measurements at 40°C and 100°C according to PN-EN ISO 3104 (ASTM D445). The laboratory calculates VI based on tables included in the standard or using mathematical formulas.

Method principle

Viscosity index is calculated from two measured kinematic viscosity values: v40 (at 40°C) and v100 (at 100°C).

Procedure A (VI ≤ 100): VI = [(L - U) / (L - H)] × 100, where L = viscosity at 40°C of oil with VI = 0 and the same v100, H = viscosity at 40°C of oil with VI = 100 and the same v100, U = v40 of test oil. Values L and H are read from standard tables.

Procedure B (VI > 100): VI = [(antilog N) - 1] / 0.00715 + 100, where N = (log H - log U) / log v100. This formula extends the VI scale above 100 without an upper limit.

Applications

• Classification of base oils (API groups I–V)
• Selection of lubricating oils for multigrade applications (engine, gear)
• Quality control of oils in production — compliance with TDS cards
• Evaluation of viscosity modifier (VII) effectiveness in formulations
• Comparison of synthetic and mineral oils
• Testing thermal stability of oils after aging (VI change)
• OEM specifications (e.g., VW, MB, BMW require VI > specified values)

Key parameters

Standard

Standard number

PN-EN ISO 2909:2002

Title (PL)

Przetwory naftowe — Obliczanie indeksu lepkosci na podstawie lepkosci kinematycznej

Title (EN)

Petroleum products — Calculation of viscosity index from kinematic viscosity

Step-by-step procedure

1. Viscosity measurement at 40°C

   Measure kinematic viscosity of oil sample at 40 ± 0.02°C according to PN-EN ISO 3104. Perform min. 2 determinations, check repeatability.

   ⏱ Time: 30–60 min • 🌡 Temperature: 40°C

2. Viscosity measurement at 100°C

   Measure kinematic viscosity of same sample at 100 ± 0.02°C. Use appropriate capillary (usually smaller diameter than for 40°C).

   ⏱ Time: 30–60 min • 🌡 Temperature: 100°C

3. Applicability range check

   Ensure v100 is in range 2–70 mm²/s (tables). For v100 > 70 mm²/s use formulas given in standard.

4. Reading L and H values from tables

   Based on v100, read from standard tables the values L (VI=0) and H (VI=100) corresponding to viscosity at 40°C of reference oils.

5. VI calculation — procedure A

   If v40 ≥ H: VI = [(L - U) / (L - H)] × 100, where U = measured v40. Round result to integer.

   ⏱ Time: 5 min

6. VI calculation — procedure B

   If v40 < H (VI > 100): N = (log H - log U) / log v100, VI = [(10^N - 1) / 0.00715] + 100.

   ⏱ Time: 5 min

7. Result verification

   Compare calculated VI with expected value for given oil type. Large deviations may indicate viscosity measurement error.

8. Reporting

   Report VI result as integer (without unit). Include v40 and v100 values and calculation method (A or B).

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• Oil bath 100°C — risk of burns, use thermal gloves
• White spirit — flammable, work under fume hood
• Glass capillaries — fragile, risk of cuts during assembly
• Oil samples — avoid prolonged skin contact, use gloves

Frequently asked questions