Factors include:

Oxidation: Reaction with oxygen, accelerated by heat, forming acids and sludge.
Thermal Stress: High operating temperatures breaking down oil molecules.
Contamination: Ingress of water, dirt, and particles causing chemical reactions.
Additive Depletion: Consumption of protective additives during operation.
Mechanical Shear: Physical breakdown of oil molecules under high pressure.
Understanding these factors helps in implementing strategies to mitigate degradation.