I read somewhere that up to, and during the 2nd world war, magnetic compasses were filled with.....whiskey! (?)
A few mechs would *drain* them, and replace with kerosene, but that quite quickly would perish the rubber seals and markings.
I think it is now a mixture that, as well as not being drinkable, will not eat away at anything.
I don't think this is an urban myth?