"Before draining the old oil, the vehicle should run for a short while so that any condensed water present can be mixed in the oil. The oil should be drained from a warm gearbox.
The oil drain plug is situated on the underside of the axle drive. Before screwing in the plug again, the magnet should be cleaned of adhering abrasives."
This is from the ZF-SYNCHROMA GEARBOX 5 DS-24 and ZF-Synchroma-Gearbox 5 DS-25/2.
This is for the original orientation gearbox not the flipped GT40. Also it recommends mild EP gearbox oils of viscosity class SAE80.