Just to close the loop, I got some proper oil and added it until the sight glasses were nearly full. The level went down the first time I ran the lathe after filling (I suppose as the oil found its way into the bearings), but it seems to have stabilized nicely in the time since.
As a bonus, with the now-clean cavities behind the sight glasses and the brand new oil, the little holes behind the sight glasses are visible. So no more guessing about what's going on there.
-- Russell Mac