Wow, this has started something. Wish it had been so clearly documented when I was having trouble back in 98-00.
Hewho's doubt over where the oil goes is understandable. For sure some is in the top end, pipes and filter and even the tank empty. When I was messing around with the original cases I never filled them to see how much they and the timing chest would hold. But, with the crank and rods in place there's a surprising volume in there + the undersides of the pistons etc. Say 0.5 litre in the filter and pipes, 0.2 litre head, 0.2 litre in the tank under the feed, 0.2 litre timing chest. That leaves 1.7 litres or so for the cases and piston insides. It's possible but it does sound a lot.....oh wait a minute that's why it pumps out of the base gasket, into the head and out of the exhaust!!
With the breather sucking air back in or each stroke as well as pumping it to the tank there will always be some extra pressure in the cases from blowby etc. The one way valve minimises that but does not stop it completely. Mine also had an intake cover breather which was eliminated once the root cause was fixed.