These gearboxes are "lubricated for life" according to the label. I guess that means that when they run out of life they are no longer lubricated.
An old one that I took off was definitely filled with heavy weight oil not grease.
Oil seems the better bet to me...its a gearbox not a bearing.
Grease tends to get thrown off the gears and sticks to the case, oil on the other hand being less viscous is always in contact with the gear teeth surfaces.
Having recently replaced both outhaul and main furling gearboxes, I hope to never have any issues.