MLG ACFT ON PREFLT SHOWED BOTH OIL QUANTITY GAUGES READING ZERO; BUT WITH TEST SWITCH DEPRESSED READ NORMAL. MAINT WAS CONSULTED WHO SAID IT COULD BE DEFERRED SO FLT CONTINUED. LATER CAPT RPTR DETERMINED THE DEFERRED QUANTITY GAUGES WERE PROBABLY ILLEGAL SINCE ONLY ONE OF THE GAUGES COULD BE DEFERRED.

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FLT WAS AN ORIGINATING AM FLT FROM CLE TO BWI. I ACCOMPLISHED THE CAPT'S COCKPIT PREFLT WHILE THE FO DID THE EXTERIOR PREFLT. I NOTED BOTH ENG OIL QUANTITY GAUGES INDICATED APPROX 3 GALS. I PRESSED THE GAUGE TEST SWITCH AND NOTED BOTH NEEDLES MOVING TOWARD ZERO. I RELEASED THE SWITCH AND CONTINUED THE COCKPIT SET UP. A FEW MINS LATER THE FO RETURNED TO THE COCKPIT AND WE ACCOMPLISHED THE BEFORE START ORIGINATING CHKLIST. WHEN WE GOT TO 'OIL QUANTITY' I SAW THAT BOTH GAUGES INDICATED ZERO. WHEN I PRESSED THE OIL QUANTITY TEST SWITCH BOTH GAUGES WOULD INDICATE NORMAL QUANTITY; BUT WHEN I RELEASED THE TEST SWITCH THE NEEDLES WOULD MOVE BACK TO ZERO. I CALLED DISPATCH AND MAINT AND EXPLAINED THE PROB. MAINT SAID THAT SINCE IT WAS THE SWITCH THAT WAS AT FAULT WE COULD DEFER THE TEST SYS AND CONTINUE TO OPERATE UNDER THE MEL. CONTRACT MAINT WAS CALLED AND VERIFIED FULL OIL QUANTITY ACCORDING TO MEL SPECIAL PROCS. FLT CONTINUED TO BWI WHERE OIL WAS SVCED AGAIN ACCORDING TO SPECIAL PROCS. ON RETURN FLT FROM BWI TO CLE I REREAD THE MEL SECTION AND CONSIDERED THE POSSIBILITY THAT WE COULD BE OPERATING IN VIOLATION OF THE MEL SINCE BOTH OIL QUANTITY GAUGES READ ZERO UNLESS THE TEST SWITCH WAS HELD IN THE TEST POS. THAT COULD POSSIBLY BE INTERPRETED AS CAUSING THE GAUGES TO BE INOP AND WE COULD ONLY DEFER 1 OIL QUANTITY GAUGE. I CALLED DISPATCH AND MAINT AND EXPRESSED MY CONCERN AND IT WAS DECIDED TO DELAY THE ACFT IN CLE UNTIL A PART COULD BE INSTALLED TO CORRECT THE PROB.