What does transformer oil inhibited mean in specifications?

Inhibited oil means the oil contains an added oxidation inhibitor to slow aging. Specifications may state the inhibitor type and target concentration range. Inhibited oils generally provide longer oxidation life and less sludge formation, particularly in transformers that breathe to atmosphere. The inhibitor can deplete and should be monitored in periodic oil analysis. Inhibited does not mean the oil cannot oxidize, it means oxidation is slowed under normal conditions. Compatibility with existing oil and transformer materials should still be confirmed.