The first fluid-filled compasses starting around 1860 were filled with an alcohol and water mixture. Compass oil was not commonly used until the 1940's and a pint of oil is only about 8 dollars from Ritchie. Danforth (AquaMeter) uses Exxon Isopar-M as the oil and Ritchie uses Isopar-L, a lower viscosity oil (older models with no label on the bottom use odorless mineral spirits). You canít use them interchangeable. You can use Ritchie fluid in a Danforth but not the other way around. Some compasses still use a water-alcohol mix using about 45% Ethyl. You can tell by smelling or rubbing the fluid between your fingers if its oil based.
Unless you can draw a vacuum put the compass in a refrigerator overnight before refilling it. Otherwise, you will always have a small bubble after resealing the compass.
Keep in mind it will need some sort of repair first, the oil doesnít just evaporate.. Danfoth will not sell parts and they own the AquaMeter brand now. Ritchie will sell the parts and a new diaphragm is as little as 12 dollars (list price) for a small Ritchie compass. Most compass repair shops will sell Danforth parts if you want to try repairing it.
All the best,