I've seen that before on a n/a Forester, except it was coolant. I think whoever worked on it prior screwed the stud in too far. While your situation is not ideal, at least the crack is not in an area of pressure. The passageway above the stud is just drain back. Your best bet is to put a good coat of rtv on the portion of the stud that threads into the head. Obviously this is just a hack, but may be enough to get you by. If all else fails, throw a cylinder head on. Ej205 heads can be found for silly cheap.