Don is correct about how water gets into the transmission and about the clutch oil.
If I remember correctly and perhaps you already know, there are two drain plugs to drain the transmission on the later models with both brake pedals on the right side. Also a separate plug to drain the hydraulics.
The second drain plug is behind the brake pedal shaft that goes under the transmission to the left brake. To clear this shaft, Case put a hump in the bottom of the transmission that resulted in pooling a small amount of transmission oil. The second drain plug drains this pool.
When draining the engine oil and the clutch housing, I usually take the left cover off the clutch housing and add a quart of new engine oil into the clutch housing because it is uncertain how rapidly engine oil moves thru the rear of the crankshaft.
The rear main has no seal so some oil leaks thru the rear main. Also, there is a drilled passage in the crankshaft that allows oil from the rear main to flow into the clutch. This drilled passage has a loose fit dowel (trapped by the flywheel) to restrict the rate that oil flows into the clutch.
If the small clearance around the dowel gets plugged with crud, then the clutch relies only on leakage around the rear main. For this reason, I put a quart into the clutch housing.
Excess clutch housing oil is slung by the flywheel into an elevated funnel of sorts that is cast into the left side of the housing.
Sorry for rambling if you already know all this.