THE ACFT COLLIDED WITH TREES DURING A FORCED LANDING, WHILE ON A DUAL INSTRUCTIONAL FLT. THE CFI SAID AFTER THE CLIMB TOCRUISE THE ENG BEGAN TO SLOW DWN. THE CFI TOOK CONTROL OF THE ACFT TO RETURN TO ARPT OF DEPARTURE. HE SAID THE VIBRATIONWAS EXCESSIVE AS THE ENG CONTINUED TO LOSE RPM. THE ENG WAS SHUT DWN AFTER THE ACFT HAD DESCENDED FROM 3500 TO 700 FT AGL. THE CFI SAID AS THE TURN TO FINAL WAS MADE IT WAS APPARENT THAT THEY WOULD LAND SHORT OF THE RWY. THE ACFT WAS LANDED IN AN OPEN FIELD BUT IMPACTED TREES DURING THE ROLLOUT. POST ACC EXAM SHOWED THAT THE OIL LINES WERE THE ORIGINALEQUIPMENT. THE ACFT SERVICE MANUAL STATES THAT THE OIL LINES SHOULD BE REPLACED AS REQUIRED, BUT NO LATER THAN 1000 HRS.THE ACFT LOG LISTS 5 ANNUALS AND ONE 100 HR INSP COMPLETED SINCE THE ACFT ACCUMULATED 1000 OPERATIONAL HRS. THE ACFT OILCOOLER'S INLET OIL LINE FAILED DURING THE TRANSITION FROM CLIMB TO CRUISE FLT. THE FAILURE RESULTED IN LOSS OF THE ENG'SOIL SUPPLY AND SUBSEQUENT ENG FAILURE.