It is a difficult situation. DEBER was still in business in Italy as late as the late1990s however they are no longer contactable.
As far as the "systems" the three separate oil tanks serve it looks like the knee tank serves the table and all axis, the top tank serves the spindle /quill and quill drive, and the main reservoir serves the horizontal spindle and perhaps lubricates the coolant pumps.
Being I see no oil filters I am assuming none of these systems are actually being pumped or circulated and probably operate in a splash style delivery system.
I did extract a small amount to look at it and by the feel test it is medium to light weight, like a 10 weight motor oil. The smell test was not nice, smells terrible, sort of a cross between rotten meat and moldy wood. it is yellowish in color and is still pretty transparent.