The general reasons of the oil leaking are: The aging of the sealing gasket, Instrument mounted on equipment vibrates frequently or excessively, the loosing of the bezel, Man-made accidental instrument falls to the ground causing the deformation of the watch cover and so on. So must find the reason of the leaking before refill the oil.
If you want to re-fill,, you have to ask to the supplier to know whether it is silicone oil or glycerin in it. Generally, there is an oil injection hole at the top of the shell of the shock-proof pressure gauge, which is sealed with a rubber plug. You can pull the rubber plug out, (take care not to destroy the rubber plug). Then you can fill the silicone oil or glycerin in and cover the rubber plug after filling.