just removed the Silva 85 compass from my boat to determine why the airbubble is growing. Found a small pool of oil under it, clearly there is a slow leak of some sort. No visble damage apparent, wonder if it is just leaking around filler screw? Could I just top off the oil (never done this before) and tighten back down? Is there some kind of gasket under the screw? Any suggestions would be most helpful. Thanks in advance!