As a quick two penneth, my oil tank used to completely empty into the crankcases within a few days before I fitted my much vaunted on-off tap, which has now solved the problem completely.
I never got any oil in the primary - it just sat in the cases until I drained it off. I did notice, however, that the crankcase oil level got so high I'm sure it managed to get past the gasket into the dynamo chain chamber as there was oil in there.
My wired bolts in the primary are not tight, nor sealed in, but no oil came through. I would imagine the crank seal is good, now that Bergs has put it in the right way round (never leaked before though, to be fair).
As has been suggested, I reckon further tests and exploration is necessary, but I'd look at the crank seal most definitely.