The chemical and physical characteristics of ionic liquids (IL) suggest that ILs when added as additives in lubricants offer better tribological properties than other conventional additives. However, the use of ionic liquids as such as a lubricant is not feasible. Phosphonium based ILs exhibit high miscibility and has been proved to be less corrosive when used as additives in small concentration. This study focuses on the behavior of Trihexyltetradecyl phosphonium bis (2,4,4-trimethylpentyl) phosphonate when blended with mineral base oil and engine oil. A comparison of its performance when used with ZDDP in diesel engine lubricant is also included. Tribological tests were carried out in a reciprocating wear test setup on AISI 52100 steel surfaces with a ball-on-flat geometry. Atomic force microscopy of the worn surfaces revealed the formation of a stable film with 3 % IL and 1 % ZDDP blend in base oil. Surface films formed with 1 % and 5 % IL and 1 % ZDDP was observed to be severely worn due to unstable film formation and corrosive nature of the IL at 5% concentration.