AbstractA novel, potential ashless S—N‐style additive, 2‐(N,N‐dibutylamino)‐4,6‐bis‐(dibutyldithiocarbamate)‐1,3,5‐triazine (DBTT), was synthesised and its synergistic tribological effect with a traditional additive, tricresyl phosphate (TCP), in rape seed oil was evaluated using a four‐ball tester. The results show that the load‐carrying capacity of the vegetable oil could be improved greatly by the presence of the TCP and/or DBTT additives; at certain mass ratios TCP and DBTT show a good synergistic effect in improving the tribological performance of the base oil. The elemental composition and chemical nature of antiwear films generated at a steel interface were investigated using Kevex energy dispersive X‐ray analysis and X‐ray photoelectron spectroscopy. Scanning electron microscopy was also used to study the rubbed surface morphology. It was found that FeS, organosulphur compound, sulphate, phosphate, and nitrogen‐containing compounds were present in the comple