it's just an o-ring

pull your dipstick out.. on the yellow plastic should be a black o-ring or a notch where an o-ring should be.. i have to replace mine too.. not sure what size it takes, but i'm sure it'd be easy enough to find. when the dipstick pops up, it's an indication that the existing o-ring has become unfit to perform its task and needs to be replaced..

or an indication that there are little gnomes in your crankcase who like to f*ck with your head

-adam