the (GFR) manual tells Agip SAE 15W50
I would use something based on mineral oil.
If you would like to use a sythetic oil please choose something which is JASO MA certified. That will make less (no) troubble with the wet clutch.
Beside this you can also use something special for 2stroker gearboxes with wet clutches.
If i remember right, mine is from Motul.