Abstract The load-carrying capacity of a base oil containing antiwear and extreme-pressure additives, sulphurized olefin (SO), dibutyl phosphite (DBP) and a thiophosphate-amine complex (SPN) was evaluated using a Timken Tester. The roughness and chemical composition of the rubbing surfaces were measured at different sliding times. A static oil immersion test of oil and steel block was carried out and the bonding strength of the protective film formed in the oil immersion test was investigated using a scratch tester. Results indicate that even with a very small amount of DBP blending with SO, a much higher Timken OK load could be obtained. The rubbing surface roughness of SO combined with DBP was much smoother than that of the SO or DBP oil alone. The scratch test of the block after immersion in the lubricant in static showed that with the combination of SO with DBP, a high-strength protective film on the steel block could be produced.