The purpose of A.C compressor oil is to lubricate the A.C system. Synthetic oil is used for R-134a A.C systems. If there is gas leakage, replacement of A.C parts such as condenser, evaporator or hose pipes etc. or servicing the A.C system, then it requires refilling with genuine compressor oil. Usage of non-genuine compressor oil can result in failure of compressor.