The oil tank is not vented directly to atmosphere but it is vented via a few things. The hose on the top of the tank is the vent hose and it goes to the Reed valve (under the triangular plate on the crankcase), into the crankcase which is vented into the throttle body inlet via a 0.100" metered orifice. There is a good illustration of the oil tank system in the S&S X-Wedge service manual page 4.8. The service manual is freely available on the MTWC website
https://www.mtwc.co.uk/new-5-speeder-page-2/
