Selecting a good lubricant: which property should be high to ensure minimal viscosity change with temperature and stable film formation?

Introduction:
Lubricant performance depends on multiple properties, but some are more universally desirable. The viscosity index (VI) measures how much viscosity changes with temperature; higher VI means more consistent lubrication over a wide thermal range. This question identifies which property should be high in a “good” general-purpose lube.

Given Data / Assumptions:

• Target: broad operating temperature conditions.
• Goal: maintain hydrodynamic film without excessive drag at low temperature.
• Other properties (volatility, pour point) have different implications.

Concept / Approach:
High VI oils thin less as temperature rises, preserving load-carrying capacity. High volatility is not desirable (it leads to evaporative loss and oil consumption). Lower pour point (not high) is desirable for cold flow; thus “high pour point” is unfavorable.

Step-by-Step Solution:
Relate VI to temperature–viscosity behavior.Evaluate alternatives: volatility should be low; pour point should be low.Conclude that high VI is the correct desirable characteristic.

Verification / Alternative check:
Data sheets for premium hydraulic and engine oils highlight high VI values (often > 140 for multigrades) as a selling point for performance stability.

Why Other Options Are Wrong:

• Volatility: high values cause losses and deposits.
• Pour point: high pour point impairs cold start flow.
• None of these: incorrect because high VI is indeed preferred.

Common Pitfalls:
Confusing VI with absolute viscosity grade; they address different aspects of lubricant selection.

Final Answer:
Viscosity index