In this study, three oil-soluble phosphonium-based ionic liquids (ILs) were synthesized and then investigated them as potential lubricating additives. The friction-reduction and antiwear performances of the ILs as lubricant additives in pentaerythritol oleate (PETO) and trimethylolpropane trioleate (TMPTO) were evaluated on an Optimol SRV-IV oscillating reciprocating friction and wear tester. The results indicated that tetradecyltrihexylphosphonium O,O′-diethyldithiophosphate (IL3=TTP−DDP) exhibited considerably better lubrication properties than commercially available tricresyl phosphate (TCP) by performing very well at 200 °C. SEM and XPS were performed to analyze the worn surfaces and revealed the fabrication of tribofilm composed of Fe(OH)O, Fe₃O₄, FePO₄ and FeSO₄, which played a crucial role in the excellent lubricating behaviours of the ILs-additived lubricants.