Rather than grease, it was probably filled with a gear oil plus a particularly incompatible additive that was used in some of the early M3W bevel boxes that could cause the oil to effectively congeal in the box. An early instruction from the factory was to get rid of this mixture as soon as possible. As for filling with actual grease I don't know if anyone has tried it but have doubts about how well it would lubricate the bearings.
