[Steve Tinker]

My point exactly. If the valve cover / cambox is flooded with oil, then the oil return to the sump is not happening.
In the "old days" on a V or in line configeration engine, the valve cover was semi-pressurised with oil & then gravity did its job of returning the oil to the sump. But on a horizontally opposed engine layout, gravity doesn't play a big part unless specifically designed oil return tubes are built into the design.
Over pressurisation / filling of valve covers and camboxes can cause all sorts of problems from simple oil leaks to blown valve stem guides, not to mention loss of crankcase oil capacity which I suspect may be happening to HS's engine......