The aim of this study was to determine the optimum formulation for vegetable oils-based synthetic ester lubricant (trimethylolpropane trioleate, TMPTO) by searching for suitable antioxidants to extend its service life. The oxidative stability was evaluated by rotary bomb oxidation test (RBOT), pressurized differential scanning calorimetry (PDSC) and oven oxidation stability test (COST). The results indicated that N-phenyl-alpha-naphthylamine (Am2) exhibited superior antioxidant activity compared to other selected antioxidants and thus could be used as the optimal primary antioxidant for TMPTO. Synergistic antioxidant effect between Am2 and a sulfur-containing auxiliary antioxidant (DLTDP) was also carefully investigated. The results suggested that the combination of Am2 and DLTDP revealed a good antioxidant synergism and could effectively improve the anti-oxidation ability of TMPTO. The best anti-oxidation result was achieved by the combination of 0.9 wt.%/0.1 wt.% Am2/DLTDP. Good antioxidant synergism between Am2 and DLTDP could be attributed to their complementary anti-oxidation mechanisms (Am2: radical scavenger and DLTDP: peroxide decomposer). (C) 2013 Elsevier B.V. All rights reserved.