I had the common problem of an air bubble in the compass as the oil leaked out. Thanks to this forum a fellow skipper described dismantling the compass. This I did and discovered a split in the base of the expansion bellows or cup at the bottom of the oil reservoir.
Such cups are no longer available as a spare part. However I have found a wonderful British product for repairing the flexible silicone cup, called Stormsure (www.stormsure.com). It is a glue that sets flexibly... can be used on sails, wetsuits inflatable dinghies, etc.Being ever flexible it is perfect for an expansionary bellows and is reportedly unaffected by oils and chemicals. It is not cheap but far cheaper than a new compass. A friend in the business suggests filling with 'white oil'... the pure clear stuff not the emulsion sprayed on roses!
I have also noted the used of baby oil suggested by another skipper.
See http://bavariayacht.org/forum/index.php/topic,1618.msg9449/topicseen.html#msg9449
Having researched the forums at length, we used Johnson's Baby Oil to top up after a helpful crewman opened the globe by mistake whilst trying to replace the compass bulbs. That was about 2 years ago and has been perfect since.
It's high grade mineral oil, but cheap and very easy to source.