Lubricants: what does the viscosity index (VI) of a lubricating oil measure with respect to temperature changes?

Introduction:
Viscosity governs hydrodynamic film formation in bearings and gears. Because machine components operate across a range of temperatures, the rate at which a lubricant thins on heating is crucial. The viscosity index (VI) is a comparative, dimensionless measure widely used for selecting oils for automotive and industrial service.

Given Data / Assumptions:

• Viscosity measured at standardized temperatures (e.g., 40°C and 100°C).
• VI is a comparative index (higher is better stability).
• No chemical composition needs to be inferred directly from VI.

Concept / Approach:
High VI oils show smaller viscosity change with temperature; they maintain film thickness at elevated temperatures and avoid excessive drag at low temperatures. VI is not a measure of flash point, nor should a “good” oil have a low VI—modern multigrade engine oils and premium hydraulic oils often have high VI due to base stock selection and VI improver additives.

Step-by-Step Solution:
Define VI: index of viscosity–temperature sensitivity.Interpretation: higher VI → better viscosity stability across temperature.Implication: improved protection over wider operating ranges.

Verification / Alternative check:
Data sheets list KV40, KV100, and VI; comparing oils shows that for similar KV100, higher VI products thin less at high temperature, matching field performance benefits.

Why Other Options Are Wrong:

• Flash point: separate property related to volatility and safety.
• “Should be low”: incorrect; higher VI is desirable.
• None of these: incorrect because option (b) is correct.

Common Pitfalls:
Equating VI with absolute viscosity grade; VI speaks to change with temperature, not the baseline viscosity at a single temperature.

Final Answer:
The extent to which viscosity varies with temperature