oil discharged from air filter

Hi sir cas. We have atlas copco air compressor GA132+FF. last month the oil returned from air filter when the compressor turn to unload situation and continue running without shut down.We stopped the compressor and disassembly the unloade valve to clean it the assembly it again,also we cleaned the air filter from collected oil.Now it works correctly but the dp air filter=-0.044 bar. what the causes of returne oil from air filter? It must be changing the air filter? thank you very much for your help:)