The nylon washer is molded onto the oil drain plug. It has a tendency to crack and fall apart over time. While I know it is a big purchase, I'd probably buy a whole new oil drain plug, or at least yank a nicer one from the junk yard.
go to autozone they will cost you like 3 bucks for a new oil drain plug and the newer ones come with a metal/rubber washer that you can reuse for a longer time than the nylon or rubber ones alone...