I think Quaife recommend every 30.000 miles - the same as the gearbox but not sure now where I red that. You may find that your car is one of the early ones where Morgan tried to quieten the box by filling it with Molybdenum disulfide sealed for life grease - mine was like this and as you say it is black. I had to warm the box up and rinse it through with white spirit several times and then use a flushing oil before filling with standard EP90 gear oil - it has done 25,000 miles since and I believe it is getting quieter but it may just be my ears shutting down!!
The front seal is like the crankshaft seal - running on a poorly finished and poorly sized shaft with the centre of the flange sealed to the splines with mastic - leaks are common. If you have the seal replaced get the shaft polished while it is out and make sure the mastic is replaced well with some under the retaining nut rather than too much inside the box.
Good luck and happy 3 wheeling,
Phil
