A B737-200 IN CLB AT FL200 DECLARED AN EMER AND DIVERTED DUE TO LOSS OF OIL QUANTITY IN #1 ENG CAUSED BY AN N2 TOWERSHAFT GEAR BOX OIL SEAL AND COVER PLATE LOOSE.

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OIL QUANTITY #1 ENG WENT TO ZERO. PRESSURE DROPPED TO ZERO AND ENG SHUT DOWN IN ACCORDANCE WITH QRH. THE ACFT WAS RETURNED TO IAD; OUR DEP POINT. AN EMER WAS DECLARED AND AN UNEVENTFUL LNDG MADE AT DULLES. MY CONCERN; ALTHOUGH AT PRESENT I KNOW OF NO ERRORS; IS THAT MY FLT MAY HAVE MISSED SOMETHING. FOR EXAMPLE; I WAS GIVEN DSCNT INSTRUCTIONS BY ATC. YET; WITH ALL THE DISTRS CREATED DURING THE SHUTDOWN OF THE #1 ENG; I WAS SLOW TO INITIATE THAT DSCNT. IN RETROSPECT; COULD THERE HAVE BEEN MORE? ONE MAJOR CAUSE FOR THIS INITIAL CONFUSION WAS THE WAY THE PROB BEGAN. I NOTICED ZERO QUANTITY INDICATED; YET PRESSURE AND TEMP WERE NORMAL. HOW COULD THIS BE? THE OIL QUANTITY CIRCUIT BREAKER WAS IN. IT TURNS OUT THAT FLT MANUAL/PLT HANDBOOK STATES THAT WITH THE QUANTITY OF ZERO; THERE IS STILL 1.3 GALLONS REMAINING. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE ENG WAS A P&W JT8D-15 AND THE LOSS OF OIL WAS CAUSED BY THE N2 TOWERSHAFT GEAR BOX SEAL AND SEAL PLATE FOUND LOOSE. THE RPTR SAID P&W WAS INVESTIGATING THIS INCIDENT.