To counter downstream oil infiltration through a shaft seal, a small portion of the compressed fluid is diverted from downstream to upstream through a passageway in a rotor. The diverted fluid is introduced to a space at a downstream side of the seal. An exemplary implementation is in a compressor having a central male rotor intermeshed with a pair of female rotors. The seal is located at an upstream (inlet) end of the lobed working portion of the male rotor.