Assuming it's a 1978 century... and assuming about an inch to an inch and a half of distilled water was added... how would one go about correcting the densities? I just did it last night (before reading this post) and now my goo just forms a large blob that hovers in the center with no flow. It looks cool, but I miss the old flow :-/ my issue was that I didn't like the space between the cap and the top of the liquid so I filled it to that line. I did add a drop of gookit surf but I figured I'll wait for some replies before I do too much more to it lol
I'm in the process of tweaking and restoring and while I've had decent results using Epsom salts and distilled water, to make a specific gravity of ~1.3 using the original goo, I think it is more complex than just topping up.
Consider that the original fluid and wax probably had carbon tet or perc in it. Even when mixed in wax, that stuff is more volatile than water and at room temperature, it should evaporate out of the lamp first, and faster.
So if your lamp is down an inch or two....what boiled out before any water did? Right, the carbon tet or perc. Possibly form the wax, not just from the fluid.
I've researched this and don't find anything on point about it. I plan to explore it slowly in weeks ahead.
But if you just want to add fluid...I'd check the specific gravity first, try to keep it where it is, if the goo moves in a way that you like right now. Change the sg and the goo will behave differently! A very small change can make it sink, or stay at the top, with no motion at all.