Bike engines crank case breathers are not as sophisticated as on car engines. HD engines are "known" for oil breather problems.
This video explains:
Breathing Oil I suspect S&S may be similar.
A significant percentage of HD owners fit a seperate oil catchpot to the breather and then let the breater vent to air. This is typical:
Breather If/when I get a M3W I'd go this route: oil on the airfilter is unhelpful.
