AFTER TAKEOFF, FOLLOWING THE INSTALLATION OF A PROPELLER GOVERNOR OIL LINE, A LOSS OF OIL PRESSURE WAS NOTICED. A PRECAUTIONARY LANDING WAS MADE IN A WHEAT FIELD. THE OIL RETURN LINE FROM THE NUMBER THREE CYLINDER WAS FOUND DISCONNECTED. THE MECHANIC WHO PERFORMED THE WORK STATED HE WAS A ONE MAN SHOP AND DUE TO DIFFICULTIES, THE JOB TOOK 3 HOURS INSTEAD OF THE PROJECTED 1 1/2 HOURS. HE ALSO SAID HE ALLOWED CUSTOMER PRESSURE TO OVERRIDE HIS STANDARDS.