A B737 FLC QUESTIONS THEIR LEGALITY AFTER MAKING A VISUAL APCH TO RWY 13 WITH THE LAST HALF OBSCURED BY FOG TOWARDS THE ROLLOUT END. RWY 35R WAS NOT LEGAL WITH AN RVR RPTED AT 2000 FT AT OKC; OK.

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VISUAL APCH TO RWY 13 OKC. WHILE READING THE NARRATIVE; PLEASE KEEP IN MIND THAT THIS EVENT HAS NEVER OCCURRED FOR ME IN ALL MY YRS OF FLYING. I WAS DISPATCHED TO OKC ON A SCHEDULED FLT. MY FLT WAS RELEASED WITH VMC EXPECTATIONS AND NO ALTERNATE LISTED ON THE RELEASE AND NO FORECAST OF LOW VISIBILITY NOTED. OKC WAS CALLING FOR CLR AND 10+ MI VISIBILITY. THE FLT WAS DELAYED FOR 20-25 MINS WITH A MECHANICAL PROB WITH THE APU IN DAL. ONCE AIRBORNE AND ENRTE TO OKC; A SIGMET WAS ISSUED BY DEP GENERICALLY TO EVERYONE ON FREQ FOR LOW CEILINGS AND VISIBILITY FROM ARDMORE NE TO TULSA AND E TO LIT. INFLT VISIBILITY WAS UNRESTR AND CLR OF ALL CLOUDS AS WE PROCEEDED N FROM THE METROPLEX. THE FOG BANK TO THE N WAS NOTED AS WE PASSED ARDMORE. ONCE WE CHKED ON TO OKC APCH; WE WERE TOLD THAT THE RVR ON RWY 17L AT OKC WAS 800 FT AND STEADY. HOWEVER; THE RVR FOR RWY 17R WAS 3000 FT VERSUS 6000 FT. WE WERE TOLD THAT WE WERE #1 FOR THAT APCH. ADDITIONALLY; THE CTLR SAID THAT IF WE EXPEDITED; WE HOPEFULLY MIGHT GET IN BEFORE THE RWY CLOSED. WE WERE CLRED TO 6000 FT AND QUICKLY BRIEFED A COMPANY PROCS APCH FOR RWY 17R. WE WERE VECTORED TO THE WEST FOR A R DOWNWIND SETUP FOR THE ILS TO RWY 17R. WE WERE BOTH VERY BUSY WITH OUR DUTIES AT THE TIME AND AS WE APCHED ABOUT MIDFIELD OKC WE ONLY NOTED THAT WE WERE IN THE CLR AND THAT A VERY DEFINED LINE OF FOG WAS VISIBLE OVER THE GND IN CLOSE PROX TO THE RWY. ON THE W SIDE OF THE FOG BANK; CLR UNRESTR CONDITIONS EXISTED. TO THE E OF THE LINE THICK LOW CLOUDS. JUST TO THE NW OF OKC; WE WERE TOLD THAT THE RVR FOR RWY 17R HAD DROPPED TO 2000 FT. WE WERE GIVEN HOLDING TO THE N ON THE LOC (RWY 17R) AT FAF. AS I TURNED ONTO THE LOC AT 6000 FT INBOUND TO FAF; I NOTICED JUST HOW CLOSE THE LINE OF FOG WAS TO THE W SIDE OF THE RWY. WITH FAF BEING 6 MI FROM THE RWY; AS I STARTED MY R TURN TO ENTER HOLDING; I LOOKED DOWN AND SAW CLRLY THE FIRST 7000 FT OF RWY 13 OKC. IT WAS COMPLETELY CLR OF CLOUDS. I TOLD THE CTLR WHAT THE CONDITIONS WERE AND ASKED IF WE MIGHT HAVE A VISUAL APCH TO THAT RWY (TO THE SIDE). A VISUAL APCH CLRNC WAS ISSUED AND ACCEPTED. THE ACFT WAS CONFIGURED FOR LNDG FLAPS 40 DEGS AND THE CHKLIST COMPLETED AS A VISUAL DSCNT TO THE W WAS PERFORMED. AS A CREW; WE DISCUSSED THE APCH AND I ASKED MY FO IF HE SAW ANY PROBS WITH OUR COURSE OF ACTION. HE NOTED NONE AND BACKED ME UP WITH A QUICK VERBAL ON THE RWY ENVIRONMENT; IE; LENGTH OF RWY; LIGHTING; AND NAVAIDS; RWY 13 HAD VASI AND IT WAS CLRLY VISIBLE. AT 1000 FT WE WERE STABILIZED ON CTRLINE AND CLRED TO LAND BY THE TWR. THE FIRST 2000 FT OF RWY 13 WERE COMPLETELY CLR WITH ONLY THE LAST HALF OF THE RWY IN THE FOG. AS I CONTINUED THE APCH I ASKED FOR RVR READING FOR ALL THE RWYS AVAILABLE TO GIVE ME AN IDEA OF WHAT THE SURFACE VISIBILITY WAS GOING TO BE ONCE WE ENTERED THE FOG BANK ON ROLLOUT. THE LOWEST GIVEN WAS 2000 FT RVR ON RWY 17L. A VMC LNDG WAS MADE ON THE 1000 FT MARK ON CTRLINE AND AS WE APCHED THE FOG BANK AT APPROX MID FIELD; JUST AS I KNEW; THE SUCCESSFUL LNDG WAS ASSURED. AT THE TIME; AS A CREW; WE FELT THAT A SAFE LNDG HAD BEEN ACHIEVED; KEEPING IN MIND THE SPIRIT AND PROCS OF COMPANY. UPON RETURN TO DAL; I IMMEDIATELY TOLD BOTH CHIEF PLTS OF MY ACTIONS. SINCE THIS HAD NEVER OCCURRED TO ME IN THE PAST; I WANTED TO BE ASSURED THAT NO PROBS LEGALLY HAD OCCURRED. AFTER A COMPLETE REVIEW OF FOM PROCS; I FELT WE HAD FOLLOWED THEM TO THE LETTER. HOWEVER; I KEPT DIGGING FOR A CLRLY DEFINED ANSWER TO THIS SIT. I AM STILL NOT SURE OF THAT ANSWER. IT WAS NOT UNTIL LATER THAT MY FO AND I DECIDED THAT MAYBE WE WOULD COMPLETE A RPT IN CASE OF AN EVENT. SUPPLEMENTAL INFO FROM ACN 537557: WE FOLLOWED THE VASI TO LNDG VISUALLY; AND UPON LNDG WE ROLLED OUT UNEVENTFULLY. THE ROLLOUT WAS COMPLETED AND APPROX 500 FT LATER WE WENT INTO THE FOG BANK; TURNED OFF AND TAXIED TO THE GATE.