2.1.2 TEST METHOD
Alkyl benzene is used as base oil because it is a synthetic
oil of the hydrocarbon family having no impurities. Thermal
aging tests are done at 120 °C for up to 450 h. The ECT of
alkyl benzene was confirmed to be less than 25 pC/ml after
the thermal aging tests [7]. Heptyl sulfide and butyl sulfoxide
are used as sulfur compounds. The concentration of these
sulfur compounds in oils is 86 ppm as converting into the
concentration of element sulfur. Sulfide compounds are
normally contained in new oil while sulfoxide compounds are
not contained in new oil and generated by the oxidation of
sulfide compounds. The addition of either of these sulfur