Microlighters

The cap on the rotary valve oil tank has a rubber; what is the function of this rubber (besides sealing)? Seems aviators like to take these rubbers for what ever reason, with the result while flying, it mess oil onto the engine as the rubber seal is not in place.

Can one replace this rubber seal with any seal, such as a "polystyrene" seal that is in oil 500ml bottles under the cap?

Isn't there a breather hole in the cap?

Looks like you've had the same problem that I did - mine also grew legs and disappeared....