My Commando 'wet sumps' if left standing and I agree that some Harley engines do the same. Upon starting the oil pump then attempts to empty the oil sump and spits it out through the breather. In the case of the Norton Commando, I drain the oil after a winter standing and then refill from the top.
Hope this helps
QED. Thst is exactly the British bike problem. Maybe it also affects less used S&S dry sump units
