The oil consumption rate of RB199s varies quite a lot. The maximum rate allowed before the engine is rejected is about 1 litre per hour, and it was this rate upon which the broad guidance of 8hrs max was written (10hrs with the slightly bigger tanks of the F3 engines corresponds to the rough same guidance). However a nice, tight, newly overhauled engine might only burn oil at a third of that rate - meaning a 24-hour sortie could be possible with careful selection of engines (not to mention selection of aircrew for arse padding and bladder size!). I would have liked a gauge to show the oil contents, rather than a caption to tell me I'd run out - and a sail for dinghy, to try and sail my way to safety if it all went wrong!