I have a W 107 350 SL 1971 with the same problem.I took of the contured rubber hose from the idle speed air distributor to the valve housing. The smoke are gone. Heavy vacum take the oil right in to the suplementary air valve .
I put a jam glass betveen the cover and the air distributor. After some minutes the was 0,2 liter of oil in the jam glass.
Arvid from Norway