An oxidation inhibitor, typically a hindered phenol, an aromatic amine or ZDDP, that sacrifices itself to break the oxidation chain reaction. While antioxidants last, the oil holds its viscosity and acid number; once they are spent, oxidation speeds up sharply and produces acids, sludge and varnish. Labs track the remaining reserve with RPVOT, FTIR and linear sweep voltammetry to predict when an oil will tip over.
Antioxidants are nearly spent, phenolics gone and amines down to 40 percent, start planning that oil change.
Oxidation is not linear. An oil can look fine for years, then go from healthy to varnishing in months once the antioxidant reserve runs out, so trend the reserve, not just acid number.
Also heard as
- oxidation inhibitor
- AO
Related terms
RPVOT
Rotating pressure vessel oxidation test, which rates how much oxidation life is left in an oil.
Varnish
Thin, sticky, amber-to-brown deposit from degraded oil that coats valves, bearings and coolers.
TAN
Total acid number: milligrams of potassium hydroxide needed to neutralize the acids in one gram of oil.
ZDDP
Zinc dialkyldithiophosphate, the classic antiwear and antioxidant additive in engine and hydraulic oils.