All hydraulic and lubricating oils are formulated with oxidation inhibiting additives. When oxidation-inhibiting additives are depleted, oil oxidation progresses, TAN raises and varnish is produced. In order to know depletion of oil oxidation inhibitors, RBOT [1] and voltammetric analysis [2] have been used. However these analyses did not tell us how much top-up of new oil is needed or sufficient. This paper demonstrates a simple method to know how much top-up is sufficient to use the oil without having varnish problems.

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