PLT OF AN SMA SINGLE ENG LAND ACFT L ENG OIL CAP OFF DURING PREFLT AND LOST OIL INFLT.

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OVER WILMINGTON; DE; MY PAX NOTICED THAT IT LOOKED LIKE WE GOT HIT BY A BIG BIRD DROPPING ON WINDSHIELD IN FRONT OF HIS SEAT (L). I GLANCED OVER AND NOTICED A YELLOW SMUDGE AT THE BASE OF THE WINDSHIELD (SMA) ABOUT THE SIZE OF A SMALL FIST. I COULDN'T GIVE IT MUCH ATTN AS I WAS DISTRACTED BY WATCHING FOR NUMEROUS ACFT IN AND OUT OF ILG ON THE 99'S PENNIES-A-POUND CHARITY AIRLIFT. I WAS TALKING TO ILG TWR WHO WERE VERY BUSY AND ABRUPT; AND I WAS FURTHER CONTINUALLY INTERRUPTED BY MY PAX WHO WAS CONSTANTLY ASKING TYPICAL 'FIRST FLT' QUESTIONS. AFTER WE PASSED ILG WITH ABOUT 9 MI TO GET TO N57; IT OCCURRED TO ME THAT THE YELLOW SPOT LOOKED LIKE OIL. I CHKED MY GAUGES; BUT ALL WAS NORMAL. I THOUGHT ABOUT WHETHER I PUT THE CAP ON THE OIL SPOUT AFTER I CHKED THE OIL DURING THE MIV PREFLT; AND COULDN'T REMEMBER IF I CLOSED IT. (SOMETHING I NEVER DID BEFORE -- FORGET TO PUT THE CAP ON. I ADDED A QUART AT MIV.) AFTER LOOKING AT THE GAUGES; WHICH WERE ALL NORMAL; THE FACT THAT I HAD 9 MI TO GET TO NEW GARDEN AND THE FACT THAT THE ILG TWR WAS SO BUSY AND AGITATED; I DECIDED TO CONTINUE TO N57. I LANDED WITH A DIRECT XWIND; STILL VERY TURBULENT; BUT THE GAUGES STILL READING NORMAL. WHEN WE GOT OUT OF THE PLANE; WE SAW OIL ALL OVER THE L SIDE OF THE ACFT AND THE CAP OFF. WE LOST 1 1/2 QUARTS OF OIL. I EITHER FORGOT TO PUT THE CAP ON; OR DIDN'T TIGHTEN IT SECURELY. I ATTRIBUTE IT TO THE WX FORECAST; MY DISTRACTING; CURIOUS PAX; AND SOME OTHER MENTAL FACTOR -- THAT MORNING I ALSO FORGOT TO SCREW MY CAR GAS CAP ON (ALSO SOMETHING I'VE NEVER FORGOTTEN BEFORE) AND WHEN I GOT HOME; I DISCOVERED I FORGOT TO PUT THE COVER ON THE OUTSIDE HOT TUB! FORTUNATELY; IT ENDED WITH NO PROBS.