I had a bulkhead mounted Danforth that had a slight leak and was developing an air bubble. There was a seal kit available for the compass but the entire thing was looking a little tired so I ended up replacing it. As I recall, the instructions were to immerse the compass in fluid to get it completely filled. you may be able to make a rubber bung and just create a damn around the fill hole.

These guys may be able to set you up with the oil http://www.vikingcompass.com/compassoil.html