As said, that is the breather/fill hole. IIRC the cap pulls off, then you un-screw.
You fill only until oil comes out the plug hole in the side.
Nothing should come out of the breather unless it is over full.
You might try taking the side plug out and see what comes out.
Here is info, see page 28....
https://s3.us-east-2.amazonaws.com/c...ervice+Man.pdf
p.s. your question would get more view/answers in the Garden Tractor section, maybe Admin can move it.