Lubricating oil formulation basics: Why are paraffinic hydrocarbons considered desirable in automotive and industrial lubricating oils? They provide a comparatively high value of which key property?

Introduction:
Lubricating oils must maintain an adequate lubricating film across a wide temperature range. One of the most critical bulk properties is viscosity index (VI), which indicates how sensitively an oil’s viscosity changes with temperature. The question asks why paraffinic hydrocarbons are especially valued in base oils.

Given Data / Assumptions:

• Lube oils operate from cold start conditions to elevated operating temperatures.
• Base oil composition (paraffinic vs naphthenic/aromatic) strongly influences VI, volatility, oxidation stability, and low-temperature flow.
• We compare broad families rather than single molecules.

Concept / Approach:
Paraffinic base stocks typically display a higher viscosity index than naphthenic or aromatic stocks. A higher VI means viscosity changes less per degree of temperature shift, helping the oil keep an adequate film at high temperature without being excessively thick when cold. While additives (e.g., viscosity index improvers) further enhance behaviour, the native VI of the base oil sets a strong foundation for performance.

Step-by-Step Solution:

Verification / Alternative check:
Handbooks show Group I/II/III paraffinic base oils with VI commonly above comparable naphthenic stocks; high-VI blends reduce the need for thickening at high temperature and improve fuel economy.

Why Other Options Are Wrong:

• Viscosity: Absolute viscosity depends on grade; paraffinic chemistry does not inherently imply “higher” viscosity.
• Smoke point: Relevant to kerosene/jet fuels, not lube oil selection.
• Pour point: Paraffins can crystallise; pour point is improved via dewaxing and additives, not inherently “high.”
• Ash content: Determined by metallic additives/contaminants, not by paraffin content.

Common Pitfalls:
Confusing viscosity (a value at one temperature) with viscosity index (temperature sensitivity of viscosity).

Final Answer:
Viscosity index