The grease must be self leveling or you will run the gears dry. I used gun grease in a steering box only to find all the grease was pushed out of the way, it was all pushed up the input shaft and in to the outer steering column tube, and the gears were dry.
Gear oils are fine if the seal and bushings are tight.
STP, and many other lubricants will also work, as long as it flows.
Exhaust pipes close to the gear box will soften the grease, which may help stiffer greases.
50/50 mix of gun grease and gear oil is also a good lubricant.
As long as it flows, even a little bit, it will work in a low speed gear box. If its stiff, it will be pushed out of the way!
