This study examined the tribological properties of the IL 1 - octyl – 3 - methylimidazolium lactate (L-L108), which is used as an additive to SAE 10W-40 and PEG400, for steel–steel contacts. The ionic liquids 1 - octyl – 3 - methylimidazolium hexafluorophosphate (L-P108) and 1 - octyl – 3 - methylimidazolium tetrafluoroborate (L-B108) were used as reference lubricants because their molecular structures are similar to that of L-L108. Tribological experiments were performed on a ball-on-disk contact tribometer for 60 min (at room temperature, with a load of 5 N, sliding speed of 0.38 m/s, and relative humidity of 30%–55%). Sliding tests showed that the lubricating performance was improved when L-L108 was used as the SAE 10W-40 and PEG400 additive. After the sliding tests, the wear scar was analyzed using white light vertical scanning interferometry and X-ray photoelectron spectroscopy (XPS). The three-dimensional images and cross-sectional line profiles showed the topography of the wear surface. When L-L108 was blended into 10W-40 and PEG400, the binding energy values of C, N, O, and Fe changed compared with a neat steel surface. The XPS analysis indicated that a tribochemical reaction occurred between the mixture and metal substrate during the friction process. This formed a chemical reaction film on the steel surface, and could have been the reason for the excellent tribological performance.