Stuart you are quite right, and I can only reiterate your comments. I wasn't aware however that the oil was hygroscopic, but do seem to remember reading something about it at some point.
Again the point made regarding amount of miles or length of time is to instill in us all the importance of regular maintenance, however my suggestion to slightly overlook this in gillyberts post was maybe a tad wrong although I'm sure it was received in the manner it was aimed at. Assuming that the oil change would take place sooner rather than later!
Temperature will have some affect on the amount of oil that can be extracted out as this will affect the viscosity of it and its ability to remain as a tacky substance that remains on the internal surfaces or a runny liquid that drains freely from the hub.
In reality the hub would never really "run out" of oil as the required amount allways remains in the hub anyway, and for those people who are certain that the correct amount went in but failed to remove it in the draining procedure then a few things could affect it as suggested, along with the possability of a small leak that doesn't really show itself just down to oil migration. There are points on the hub that WILL allow oil to migrate from that have no oil seals but really solely on an interference fit with no seal etc and has at some point been the topic of conversation before regarding the hub, although this shouldn't be a cause for concern, its just the nature of the beast.
The slightly over rated term that the hub sits in a "bath" of oil comes to be realised when its more of a small spill really! I think it just sends a placebo picture to the mind of all those gears enjoying a deep soothing and relaxing soak in all seasons when in reality its nothing more than a small drip. However without wanting to cause dismay and lucid thoughts of dried up gears as long as there is 15ml in the hub the gears are ecstatic with lubrication.
Dave.