These gauges might be filled with either glycerine, mineral oil, or food grade silicone oil. Basically, something inert, that if it leaks into the water side of the gauge will not be toxic. If the "oil" in the gauge dissolves in water--it is glycerine. I can't think off hand of a quick and easy test to differentiate mineral oil from silicone oil.
My preference for a watermaker pressure gauge would definitely be glycerine. Mineral oil and silicone oil would destroy the membranes if they ever leaked into the water side even if they did not poison the resulting drinking water.
Charleston, SC, USA