The reason for that sticker is that back in the moped heyday there was camping fuel called "white gas" and it was unleaded gasoline. It was not designed as a motor fuel and could cause damage if used as such. The sticker was to prevent cheap-ass moped owners from riuning their machines then blaming the manufacturer for not telling them so.
As far as additives go, don't use them. They are formulated for 4 stroke engines and can harm a 2 stroke. A good quality 2 stroke oil is all you need.