CESSNA PILOT REPORTS OIL ON WINDSCREEN DURING DEPARTURE AND DECLARES EMERGENCY WITH TOWER AND RETURNS FOR SAFE LANDING.

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CLBING OUT FROM RWY XXL; I NOTICED OIL DROPS ON THE WINDSHIELD. I HAD JUST STARTED TURNING TO MY VECTOR HDG AND THE CTLR HAD JUST INSTRUCTED ME TO CONTACT DEP. THE OIL DROPS QUICKLY TURNED INTO A SOLID COVERAGE OF A CIRCULAR AREA OF ABOUT 8-10 INCHES IN DIAMETER RIGHT IN FRONT OF MY VIEWING AREA. SINCE I HAD ADDED A QUART OF ENG OIL BEFORE THE FLT; I IMMEDIATELY ASSUMED IT WAS RELATED. NOT KNOWING THE EXTENT OF THE OIL LEAK; I DECIDED TO DECLARE AN EMER; WHICH I DID ON THE TWR FREQ. I WAS STILL IN VMC; WELL BELOW THE 2300 FT OVCST LAYER AT THIS POINT. I TOLD THE TWR I HAD AN ENG EMER; BUT THAT THE ENG WAS STILL MAKING GOOD PWR. THE TWR CLRED ME TO LAND ON RWY XXR. I BEGAN A TURN FOR A CLOSE-IN R DOWNWIND TO RWY XXR. HOWEVER; SEEING THE 12000 FT LONG RWY RIGHT BELOW ME; I REQUESTED AND RECEIVED CLRNC TO LAND OPPOSITE DIRECTION. I LANDED BEYOND MID-FIELD AND REQUESTED CLRNC TO BACK-TAXI TO THE TXWY WHICH WAS A FEW HUNDRED FT BEHIND ME. TWR CLRED THAT AND I EXECUTED THE TURN AND BACK-TAXI. THE ENG CONTINUED TO PRODUCE NORMAL PWR THROUGHOUT THE APCH; LNDG; AND TAXIING. I TAXIED BACK TO THE FBO AND WAS MET THERE BY EMER PERSONNEL. ONE OF THEM OPENED THE OIL FILTER ACCESS DOOR ON THE COWLING AND RPTED THAT THE OIL FILLER CAP HAD COME OFF INFLT. UPON EXAM; THE OIL LEVEL WAS ABOUT 1/4 - 1/2 QUART LOWER THAN IT WAS BEFORE THE ENG WAS STARTED. I BELIEVE THE ROOT CAUSE OF THIS EMER WAS THAT I FAILED TO SECURELY REPLACE THE OIL FILLER CAP AFTER ADDING ENG OIL DURING THE PREFLT. CURRENTLY; CESSNA'S PUBLISHED PREFLT PROCS CALL FOR CHKING THE ENG OIL LEVEL (WHICH IS DONE VIA A DIFFERENT COWLING ACCESS DOOR); BUT THE PREFLT PROCS DO NOT CALL FOR CHKING THE ENG OIL FILLER CAP. I WILL BE ADDING THIS ITEM TO MY PERSONAL PREFLT PROCS IN THE FUTURE. I HAVE RECONSIDERED WHETHER I SHOULD OR SHOULD NOT HAVE DECLARED AN EMER IN THIS SITUATION. THE FACT IS THAT I WOULD HAVE BEEN FINE HAD I SIMPLY REQUESTED AN URGENT RETURN TO THE FIELD; FLOWN A FULL PATTERN BACK AROUND AND LANDED. AS IT WAS; THE RWY WAS CLOSED FOR SEVERAL MINS WHILE I LANDED AGAINST THE FLOW AND EMER CREWS SWEPT THE RWY; WHICH RESULTED IN ABOUT A 20 MIN DELAY FOR 3-4 AIRLINERS WAITING AT RWY XXR. NONE OF THAT WOULD HAVE HAPPENED HAD I NOT USED THE 'E' WORD. HOWEVER; I BELIEVE I DID THE BEST THING BY DECLARING THE EMER. AT THE TIME; I DID NOT KNOW THE ROOT CAUSE OF THE OIL ON THE WINDSHIELD AND EVEN WITH THE LIMITED OIL LOSS; THERE STILL COULD HAVE BEEN AN ENG FIRE. THINKING OF OTHER PROB IN A SINGLE ENG ACFT IS JUST ALWAYS AN EMER!