The work aims to meet energy conservation requirements by selecting low viscosity lubricants as far as possible, and further explore friction and lubrication mechanism of ionic liquid as an additive. Low viscosity polyalphaolefin (PAO-4 and PAO-8) was chosen as base oil, and zinc dialkyl dithiophosphate (ZDDP) and quaternary phosphonium salts of oleic acid ionic liquid (IL) were chosen as additives. Friction coefficient was measured at 40 ℃ and 100 ℃, respectively on friction and wear four-ball tester. Diameter of wear scars was observed with a microscope and also compared, the surfaces of wear scars were analyzed with SEM and EDS. Surface 3D scanning and roughness analysis were completed with the aid of a contact roughness profiler. Compared with the base oil, ionic liquid can effectively reduce friction coefficient, while traditional additive ZDDP increases friction coefficient. In some working conditions especially in high temperature environment, both ionic liquid and ZDDP can effectively reduce wear. According to EDS and surface roughness analysis results, it can be concluded that ZDDP and IL produce two distinct kinds of friction reaction film on friction surface, thus causing differences in friction performance. Film forming mechanisms of the films are further discussed.