For preparing an artificially oxidized oil sample, test oil was oxidized by ASTM 2440-99 [11], with some modifications, as follows: 500 mL of test oil was heated to 110 ± 0.5 C, and mixed with 1.5 L/h of 99.4% pure oxygen in the customized oil apparatus, as shown in Fig. 3. After the oxygen rushed into the drying tower, it entered the micro-bubbler and immediately contacted the copper coil (180 cm long). The oil was stirred at 300 rpm for uniform oxidation of the oil.