An oil lubricated common rail diesel pump includes a pumping assembly and a drivetrain assembly. The pumping assembly includes a pump housing and a plunger mounted along a pumping axis. The drivetrain assembly includes a driveshaft and a cam mounted within a first chamber of a drivetrain housing. The plunger is arranged for reciprocating linear movement along the pumping axis within a second chamber of the housing upon rotation of the cam. The drivetrain assembly also includes a guide mounted within the housing between the cam and the plunger and being adapted to receive a cam follower. At least the housing is adapted to be substantially filled with oil in use and the guide includes at least one flow passage communicating between the first chamber and the second chamber.