A B747-200 IN CRUISE AT FL330 DIVERTED AND SHUT DOWN #3 ENG DUE TO LOSS OF OIL PRESSURE CAUSED BY A FAILED O-RING SEAL IN THE OIL DISTRIBUTION SYS.

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AFTER APPROX 2 HRS 45 MINS AFTER TKOF; #3 ENG OIL PRESSURE STARTED FLUCTUATING BTWN 40-50 PSI CONTINUOUSLY. IT OCCASIONALLY DROPPED TO 34 PSI (RED RADIAL) CAUSING OIL PRESSURE LOW LIGHT TO COME ON (OIL TEMP WAS STEADY AT 72 DEGS C AND MATCHED OTHER 3 ENGS. CONTACTED MAINT AND DISPATCH WHO DIRECTED RETURN TO MIA. AT XA30Z; PRECAUTIONARY SHUTDOWN ENG AS PRESSURE DROPPED BELOW RED RADIAL EVERY 5-10 SECONDS. WE HAD NO IDEA OF OIL QUANTITY AS QUANTITY GAUGE WAS MEL'ED (DEFERRED). OIL QUANTITY WAS PHYSICALLY CHKED BY MAINT BEFORE FLT PER MEL. AS CREW; WE DETERMINED THAT RETURN TO MIA WAS AS SAFE AS DIVERTING TO ANY OTHER ARPT IN AREA. WE HAD NO WX OR NOTAMS FOR OTHER ARPTS. WE HAD WX AND NOTAMS FOR MIA; AND IT WAS A FAMILIAR FIELD. THE ENG COULD BE RESTARTED IF NECESSARY. IDLE OIL PRESSURE WAS MAINTAINED DUE TO WINDMILLING FAN BLADES. ATC ADVISED OF PRECAUTIONARY ENG SHUTDOWN. SPECIAL ATC SVCS WERE DECLINED. NO EMER WAS DECLARED. ALL APPROPRIATE CHKLISTS COMPLETED OR REVIEWED. ENG PROB WOULD HAVE BEEN DETECTED A LOT EARLIER IF THE OIL QUANTITY GAUGE HAD BEEN WORKING. CRM WORKED. EACH MEMBER OF CREW CONTRIBUTED TO SUCCESSFUL RESOLUTION OF PROB BY WORKING AS A TEAM AND BY MAKING INDIVIDUAL CONTRIBUTIONS TO THE DECISION MAKING PROCESS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE LOSS OF OIL WAS CAUSED BY A FAILED O-RING SEAL IN THE OIL SYS SOMEWHERE. THE RPTR SAID MAINT ADVISED AN O-RING SEAL HAD FAILED BUT DID NOT RPT THE COMPONENT OR LINE. THE RPTR SAID NO WARNING OF THE LOSS OF OIL WAS POSSIBLE AS THE #3 ENG OIL QUANTITY INDICATOR WAS DEFERRED AS INOP.