FLC OF AN MLG CANCELED IFR AND WERE CLRED FOR A VISUAL APCH EVEN THOUGH THE LATEST WX RPT INDICATED THE ARPT WAS IFR. THE FLC SUBSEQUENTLY LOST SIGHT OF THE ARPT AND CONTINUED VIA AN ILS APCH WITHOUT AN ATC CLRNC.

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WE ACCEPTED; AND THE CTLR ISSUED A VISUAL CLRNC TO RWY 29 WHEN XA45Z WX WAS IMC; CEILING 20 BROKEN VISIBILITY 1 1/2 MI. ON DOWNWIND THE CAPT (PF) INFORMED ME THAT HE HAD THE FIELD IN SIGHT AND ASKED ME TO CALL IT IN SIGHT TO THE CTLR. I INDICATED THAT THE FIELD WAS IFR SO THAT THE CTLR COULD NOT ISSUE A VISUAL APCH. THE CAPT INDICATED THAT THAT DIDN'T MATTER SO LONG AS WE HAD THE FIELD IN SIGHT OR THE PRECEDING ACFT IN SIGHT. THE CTLR POINTED OUT OUR PRECEDING TFC WHICH WE SAW. AS INSTRUCTED BY THE CAPT; I CALLED THE TFC AND THE FIELD IN SIGHT. THE CTLR CLRED US FOR THE VISUAL APCH TO FOLLOW PRECEDING TFC. ONCE ON FINAL WE DID LOSE THE PRECEDING TFC AND THE FIELD AND THE CAPT FLEW THE ILS EVEN THOUGH WE WERE CLRED FOR THE VISUAL AND THE FIELD WAS IFR. ONCE ON THE NEXT LEG I PULLED OUT THE COMMERCIAL BOOK AND POINTED OUT THAT 3 MI VISIBILITY AND CEILING 1000 FT WERE NECESSARY TO SHOOT THE VISUAL APCH. HE CONCURRED AND ADMITTED HE HAD NOT KNOWN THAT WAS NECESSARY IF THE FIELD OR THE PRECEDING ACFT WAS IN SIGHT. WE BOTH THOUGHT THE FIELD WOULD REMAIN IN SIGHT THROUGHOUT THE APCH. IT WASN'T UNTIL WE WERE ON A 5 MI FINAL THAT WE REALIZED IT WAS FOGGIER THAN IT LOOKED ON DOWNWIND. I FELT IT WAS SAFER TO COMPLY WITH THE CAPT'S INSTRUCTIONS AND ARGUE ABOUT REGS ON THE GND SINCE I; TOO; THOUGHT IN SPITE OF THE LATEST WX OBSERVATION THAT THE FIELD COULD BE KEPT IN SIGHT. WHEN WE LOST SIGHT OF THE FIELD WE WERE ALREADY TALKING TO OAK TWR AND SHOULD HAVE INDICATED THAT TO THE TWR AND REQUESTED THE ILS OFFICIALLY.