The top casting of the Super A steering gear housing is rather like an inverted cup, or a diving bell. In the top center of the bell is a cast retainer for the sector shaft bearing. The sector shaft is vertical so the bearing lies horizontally. The grease zerk is positioned just above the open bearing and lubes the bearing. If oil were used it would fall right through the bearing.
The top plug is the fill plug for the worm/sector oil. The middle elevation plug on the aft side of the upper casting is the oil level indicator and lies about halfway up the worm/sector in elevation. The lowest plug on the right side of the casting is the drain plug. The joint/seal between the upper casting and lower casting (from which projects the steering center arm) keeps the oil in.
In the center of the lower casting is the passthrough/support for the lower portion of the sector shaft. This casting extends upward to about the level of the side fill plug, meaning that oil cannot leak out down through the sector shaft bushing as long as the oil isn't overfilled. The photo of your steering gear shows that oil has been leaking out between the upper and lower castings. I don't know if there is a gasket between upper and lower castings, or whether it's a metal to metal fit. If there's no gasket, Form-a-gasket should work.
This description does imply that excess grease from the top sector bearing WILL makes its way into the oil supply.
