Auto Transmission Woes in F-Series

G'day all. We are looking for any one who may be able to shed some light on a weird problem our F truck seems to have. The auto transmission oil level dropped to below showing anything on the dip stick while we were on the Gibb River Road, and the vehicle started hunting through gears, plus it registered a fault code. We managed to get to Kununurra with what oil we could beg borrow or steal (the vehicle had just been serviced so we didn't think to have any spare on board...big mistake!) After putting around 4 lt into the transmission which has an aftermarket larger sized pan, we have noticed each time we check it, that the level has fallen to below registering on the stick again. To date, we have poured nearly 8 more lt into it with no obvious leaks anywhere. We are now in Darwin and the vehicle is in the Ford workshop, and even they can't shed any light on where this extra oil is going! The obvious wouldbe to say that the mechanics who installed the larger sizes sump have not filled it correctly, but as this was done by a VERY professional workshop, I would say definately not the case.
Any clues? Cheers, Jan and Easy