Michel,
1) No you are not going blind, there is no drain plug for the crankcase chamber! As the EW is a total loss oiling system, the only oil retained in the engine is a residual amount that had not yet escaped past the piston rings. So this will be a small amount in the crankcase, and a certain level in the timing chest sufficient to lubricate the timing gears. The level in the timing chest is controlled by a standpipe that exits out the timing chest floor. This leads over to the primary chain, and also serves as the engine breather. The oil standing in the bottom of the timing chest also feeds the second stage of the oil pump before being pumped into the crankshaft, so do not start off with the timing chest drained.
Since you never change the oil, rather just top up the oil tank as the oil is consumed (or leaks out), Douglas never saw fit to provide a means to drain the crankcase. The greatest quantity you could have in the engine would be the contents of the oil line leading down from the sight-glass having drained into the engine, or over priming the engine with the hand pump.
2) I do not know if the oil compartment capacity was ever stated. Basically you fill it up to near the top, same as you would do for petrol.
3) Straight 50W would be closer to the original. I would not recommend 20W50, as once the engine is hot you would be feeding it relatively cool 20W oil, which would be a little thin and might escape past the rings too easily. Granted once in the engine it should heat up very quickly and be more like 50W, but the oil is introduced to the engine right at the front piston skirt (after a quick trip through the first stage of the oil pump.) However in a pinch, practically any motor oil will work, and I have used muti-weight to top up while out on the road. There is even one historic instance of mustard seed oil having been used! Do not bother using synthetic oil, it will not be in the engine long enough to warrant the extra cost.
-Doug
