A set of piston compressors (2, 4, 6, 8) for use in a refrigeration system comprises a fluid connection (10) between the crank cases of the piston compressors (2, 4, 6, 8) for allowing oil compensation between said piston compressors (2, 4, 6, 8); and a control unit (12) which in operation monitors the running time of each piston compressor (2, 4, 6, 8) and, in case all piston compressors (2, 4, 6, 8) are running, subsequently interrupts the operation of a respective piston compressor (2, 4, 6, 8) for a predetermined period of time for oil compensation if its running time reaches a predetermined maximum running time.