My '41 gear box doesn't have that hole, it's not a vent. I use straight Corn Head grease from John Deere. Tractor supply has steering gear lubricant, or you can use CV joint grease used in front wheel drive cars. Grade 0 or 00 self leveling grease. I poured the grease out of the tube into a heavy plastic bag, then cut the corner off and squeezed it into the gear box. The more you move it around, the runnier it gets.
