Lubrication fundamentals — primary factor changing engine oil viscosity in service What is the main cause of change in the apparent viscosity of engine oil during operation?

Introduction / Context:
Viscosity strongly influences hydrodynamic film strength and friction. Understanding what primarily changes viscosity in real time helps explain cold-start wear and hot-idle oil-pressure behavior.

Given Data / Assumptions:

• Multigrade oil operating through cold start to fully hot conditions.
• Normal engine free of catastrophic dilution or sludge.

Concept / Approach:
Viscosity is temperature-dependent: as temperature increases, oil thins (viscosity decreases); as temperature drops, oil thickens. Although contamination can degrade oil over long intervals, the immediate and dominant in-service change in viscosity is due to temperature variation.

Step-by-Step Solution:
Consider cold start: low temperature → high viscosity → higher cranking torque.After warm-up: high temperature → lower viscosity → lower oil pressure at idle.Therefore, temperature is the main factor altering viscosity during normal operation.

Verification / Alternative check:
Oil viscosity indices and SAE grades define performance across temperature ranges, confirming the primary temperature effect.

Why Other Options Are Wrong:

• Humidity: has negligible direct effect on oil viscosity inside a sealed engine.
• Vibration: does not materially change viscosity.
• Contamination: changes viscosity over time but is not the main immediate cause during regular operation.

Common Pitfalls:
Confusing oil-pressure changes with pump faults; often they track oil temperature and viscosity, not pump failure.

Final Answer:
temperature