Which oil for 1951 Airone Sport?

Just realised I should have put this topic here and not on general questions,its the first time Ive used this site. I`m putting my Airone on the road after a long time in storage,what is the current view on engine oil,is it best to stick with tradition and use staight 30/40 or use a modern multigrade 20/50 such as Castrol XXL?