How often should transformer oil be changed under normal operating conditions?

Under normal conditions, transformer oil is not replaced on a fixed calendar like engine oil. Utilities rely on condition based maintenance using trends from moisture, acidity, dielectric losses, and DGA. Oil may last decades if kept dry and stable. Replacement or regeneration is considered when acidity rises, interfacial tension drops, tan delta worsens, moisture remains high despite drying, or DGA indicates abnormal fault activity. Practical decisions also depend on transformer age, criticality, leak history, and whether oil treatment can restore performance without full replacement.