I find that no matter how you try to seal the sector gear housing from leaking oil, it fails. Over the years I have found that first adding grease to seal the sector housing, then adding 75W145 GL4 gear oil, leakage is minimized. I drive my Model A a few thousand miles per year, and my technique works.
Model "A" service bulletins have numerous warnings about not using grease in the steering gear box because it did not have enough squeeze-out resistance to keep the gears coated. I suspect this warning was due to the animal fat greases of the time (pre WW2). Greases made primarily of animal fat turned rancid and lost their lubricating property. Modern chassis greases have superior squeeze-out resistance, and I find that they are suitable for use in the steering gear box.
The original designs of the gear box housing and the sector housing had grease fittings in them. However, these fitting were eliminated in 1928. I am adding the grease fitting back into the sector housing, so I can pump grease in-between the sector gear bushings. To do this, I am reversing the attached service bulletin.