Lubricating oils: In refinery and lube-blending practice, a “multigrade” lubricating oil refers to an oil formulated to maintain suitable flow at low temperature and sufficient film strength at high temperature, which effectively means an oil having a high viscosity index.

Introduction / Context:
Multigrade lubricating oils are designed to perform across a wide temperature range, flowing adequately when cold while still protecting components when hot. The key property that enables this dual behavior is a high viscosity index, supported by base oil selection and viscosity index improvers.

Given Data / Assumptions:

• Term: “multigrade” as used in engine and industrial lubricants (e.g., SAE 10W-30).
• We compare several oil properties: viscosity, aniline point, flash point, and viscosity index.
• Goal: identify which property being “high” characterizes multigrade behavior.

Concept / Approach:
Viscosity index (VI) expresses how little an oil’s viscosity changes with temperature. A higher VI means the oil thickens less at low temperatures and thins less at high temperatures, enabling one formulation to cover multiple “grades” in standardized classifications.

Step-by-Step Solution:

Verification / Alternative check:
SAE multigrade oils achieve low-temperature cranking limits and high-temperature kinematic viscosity targets with polymers and high-VI base stocks, confirming the role of a high VI.

Why Other Options Are Wrong:

• Viscosity: A single viscosity value does not capture behavior across temperatures.
• Aniline point: A solvency/aromaticity indicator, not a multigrade criterion.
• Flash point: Relates to volatility and safety; does not define multigrade behavior.

Common Pitfalls:
Confusing absolute viscosity at one temperature with viscosity-temperature stability; multigrade is about the latter.

Final Answer:
Viscosity index