a cam shaft has a tubular part extending longitudinally along an axis and a helical body fixed in the tubular part and having a first helical passage extending between an inlet end where a first pressure is present and an outlet end where a second pressure is present . this helical body forms an oil separator between the inlet and outlet ends . a flow - blocking element at the inlet end moves as a function of a pressure differential between the first and second pressures to open and close the first helical passage .