As a guess Mal, if there were no remains of grease in the box, then possibly stick with oil.
Maybe an EP 80 or 90 grade?
Personally, I would rather have oil in the box than grease, as it will always be in contact with the gears when moving. Whereas grease will get slung to the side of the casings and just sit there.
Just guesses on my behalf as I use a constant drive 'blades of death' machine....