I think I've got it now. When there is a slow leak in the system it causes a reduce oil level in the reservoir/gearbox (which is where the oil level sensor is). The remaining oil is still "transiting through" the reservoir/gearbox but not hanging around long enough to give a meaningful reading. Thus the lubrication system is still operating with a much reduced indicated oil level. So it's really a case of "partial oil loss" as clearly with a total oil loss there would be no oil to lubricate the bearings at all. I wouldn't expect the situation to last that long,eventually the oil would leak away to a point where lubrication would end. this ties up neatly with RMC's statement "Our company does not have a procedure for monitoring secondary instruments ( oil quantity, pressure and temperature on the basis that " if it becomes critical the aircraft will tell us"
