OK...what you've got with that ST is either a sealed front bearing that isn't sealing anymore OR a worn fit between the plain part of the crankshaft (in front of the inlet) and the crankcase.

At highish revs there's an average positive pressure in the crankcase which forces oil out the front bearing to keep it lubricated. At idle, the average crankcase pressure is negative so it can draw air past the front bearing and into the crankcase. In any engine where the clearances for oil are controlled properly there won't be enough air drawn past the front bearing to keep an engine running. Try fitting a new sealed bearing to the front (personally, if it's sealed both sides I'd remove the seal from the rear face to allow engine oil to get to it).