What is inhibited oil used in transformers?

Inhibited oil contains oxidation inhibitors, commonly additives that slow oxidation reactions at operating temperatures. The inhibitor helps reduce acid formation and sludge buildup, extending oil life and maintaining dielectric properties longer under comparable conditions. Inhibited oils can be beneficial in transformers with higher thermal stress or where long oil life is desired, but inhibitors can deplete over time and may require monitoring. Utilities select inhibited or uninhibited oil based on design practices, maintenance philosophy, and compatibility with existing oil in the fleet.