Factors determining oil quality include:

Dielectric Strength: Ability to withstand electrical stress.
Moisture Content: Low levels are essential for insulation performance.
Acidity (Neutralization Number): Indicates oxidation and the presence of corrosive compounds.
Viscosity: Affects flow and heat dissipation.
Particle Count: Reflects cleanliness.
Quality is measured through standardized laboratory tests that assess these parameters, comparing results to industry standards or manufacturer specifications.