What was found
AT FL 200, NR 3 ENGINE OIL PRESSURE GRADUALLY LOWERED FROM 40 PSI TO 30 PSI. ACCOMPLISHED ABNORMAL CHECKLIST AND INFLIGHT SHUTDOWN. MAINTENANCE REMOVED AND REPLACED PRESSURE X-MITTER, OIL PRESSURE INDICATOR. OPS CHECK GOOD. SWAPPED NR 2 AND NR 3 OIL PRESSURE IND FOR TROUBLESHOOTING, SWAPPED NR 3 AND NR 1 OIL PRESSURE TRANSMITTER FOR TROUBLESHOOTING. SWAPPED OIL PRESSURE X-MITTER HARD LINE WITH NR 3 TO NR 4 ENGINE. OPS CHECK AND LEAK CHECK, ALL ENGINES GOOD ON GROUND ENGINE RUN, NO DEFECT NOTED.