I thought I would post the under-seat label from my US model K2, a 1985 bike:
 
As you see, it recommends a 20/50 oil (or "sintetyc"!).  I have been using Miller 20/50 semi-synthetic over the summer and I am intending to use a standard 10/40 semi for the winter, when it probably won't see a lot of use anyway, depending on the weather.  I tend to give up when cleaning time starts to exceed riding time!  This is the oil strategy I have been using for my Bonneville and Trident the last few years and it seems to work for me.  I like to think the Bonnie is a bit easier to kick over in winter with the slightly lighter oil, but that's probably in my imagination!  With its 11:1 CR, the 350 Morini isn't much easier to kick over than the 750 Bonneville 
 
 
Incidentally, I have never had any clutch slip problems with wet clutches and semi-synthetic and I do wonder if this is a bit of a myth.  Very little oil actually penetrates inside the clutch plate stack.  Not an issue for the Morini anyway!