An oil soluble multifunctional protic ionic liquid (IL) was synthesized and its tribological and antioxidant properties in poly alpha olefin (PAO4) were investigated. The tribological results demonstrated that the IL significantly reduced friction and wear of PAO4. The PAO4 blend with IL resulted in an induced oxidation time of 555 min which is 8.2 and 3.5 times higher than that of pure PAO4 and PAO4 with zinc dialkyl dithiophosphate (ZDDP) for the rotating pressure vessel oxidation test. It is likely that free nonylated diphenylamine acted as a radical scavenger to enhance antioxidant performance, while free bis(2-ethylhexyl) phosphate was more prone to adsorb and react with the metal surface to form a phosphorus-rich tribofilm in order to protect the rubbing surface.