Variable Load asks a very pertinent question:
FH1100, how do you have a reliable oil quantity gauge when almost all of the oil is in "circulation" when the gearbox is running?
Answer: I don't know. I'm a pilot, not an engineer.
But heck, there has to be some way. As far as I know, all helicopter transmissions are "wet-sump" designs. That is, all of the oil is contained in the unit itself, with no external tank. I wonder if it is totally impossible to design a transmission with some sort of external sump? Alternatively, naively, I would think that there has to be some way of measuring the oil quantity in the trans and displaying that info to the pilots. I'd be very surprised if it simply was impossible.