Zac,
Quite reliable. It is a two-stage, fixed displacement pump. Short of the gear stripping or the peg for the cam shearing (not seen happen) there is nothing to halt the mechanical operation. Nor have I seen one with a worn bore. About the only think that can go wrong in the pump itself is there is a spring loaded ball across the top pump bore that acts as a bypass. As you screw down the needle valve in the sight glass, more oil has to escape via the bypass back to the inlet side of the pump.
If the spring has weakened, broken, or the ball stuck off the seat then oil will be inclined to take the bypass, than go through the sight glass.
As for how much oil, I have never seen a specified value from the factory for the SW/DT models. I should think the typical vintage 'slight haze' from the exhaust when you give it some throttle would be apt. There should only be a small amount as you described when draining off the sump. Since it is a total loss oiling system, all the oil that is delivered to the engine has to either escape by the piston rings and be burned or otherwise leak out.
-Doug
