This subject may have been touched on before, but I can recommend MTL gear oil for later Land Rover main boxes (Normally filled with ATF). I have been trying it for some time in a Discovery 1 with a very graunchy gearbox and it has made it a much nicer, quieter box to use. It is expensive and seemingly only available in small bottles, but cheaper than a rebuilt box if things are not beyond repair. Available from the big auction website.