It is useful and important to find out what is in the compass already. It could be mineral oil
, it could be kerosene, it could be mineral spirits and it could be a blend.
You can replace whatever is in the compass but you should know what the materials of the compass are made from so that you have no risk of damaging any internal components, compass card, etc. Some fluids can also be mixed but yuo gotta know what you are doing.
You can flush the compass with isopropyl alcohol or the new replacement fluid.
Here is an MSDS on the Airpath compass fluid.
I hit the West Marine
catalog for compass fluid and the Ritchie stuff (Isopar L) came up for $12.99 a pint. The aircraft version is $1.65 a pint.
I doubt that the Isopar stuff is worth $11 more per pint...
AIRPATH COMPASS REPAIR KITS - COMPASS FLUID: 1/2 PINT & QUART from Aircraft Spruce
In regards to the "leak" - If it is an external leak you'll want to get it fixed. If it's just the o-ring on the filler or something you could be ok just filling it. Unless I was getting visible wetness and the compass was OK otherwise, I'd just fill it and move on.