Detecting DBPC in transformer oil is generally a positive sign, because it means the oil is still inhibited and has some remaining oxidation protection. However, the key issue is not the presence itself, but the actual concentration. Fresh inhibited oils are typically around 0.3% by weight, and when the level falls to about 0.1% or lower, re-inhibition is usually recommended. It is also important not to overdose, since excessive inhibitor can cause other problems. In practice, DBPC should be interpreted together with acidity (TAN), interfacial tension, resistivity, sludge tendency, and DGA. Good sealing and low oxygen ingress are just as important as inhibitor level for long-term oil stability.