A DC9 TECHNICIAN RPTS DEFECTIVE DESIGN OF OIL TANK FILLER CAP WAS CAUSE FOR LOSS OF ENG OIL AND DIVERSION.

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I'M WRITING IN RESPONSE TO WHAT I CONSIDER A PRODUCT DEFECT IN THE PRATT & WHITNEY JT-8 ENG OIL FILL CAP LOCKING MECHANISM. IT IS WELL KNOWN AMONG LINE MECHS THAT THE LOCKING HANDLE CAN VISUALLY APPEAR TO BE SEATED IN THE FULL DOWN AND LOCKED DETENT. WHEN ACTUALLY IT IS NOT! THE HANDLE IS TOO SMALL TO EFFECTIVELY LEVERAGE TO THE LOCKED POS WITH THUMB AND FOREFINGER ONLY; ESPECIALLY IN WINTER OR COLD CLIMATES. SUCH WAS THE CASE I BELIEVE ON APR/THU/02 IN WHICH I PROPERLY SVCED L AND R ENG OILS; THEN SECURED BOTH ENG OIL FILL CAPS. UPON DEP; THE CREW NOTICED A DECREASE IN R ENG OIL LEVEL AND SAFELY DIVERTED TO ZZZ1 WHERE; ON INSPECTION OF THE R ENG OIL FILL CAP; IT WAS FOUND SLIGHTLY DISLODGED. THIS CAP NEEDS EITHER A REDESIGN OR MODIFICATION TO PREVENT A MUCH MORE SERIOUS EMER IN THE FUTURE. A LARGER LOCKING HANDLE WITH POSITIVE LEVERAGE TO THE FULL DOWN AND LOCKED POS IS ALREADY IN USE AT OTHER AIRLINES AND ON THE DC9'S APU. THE COST IN DOLLARS TO FIX THIS; SIMPLY DOESN'T MATCH THE POTENTIAL FOR DISASTER.