Some 'peds take 50:1, some 40:1 and yet others take 33:1 (like my 1977 Minarelli-engined vehicle --- although there's even some debate about that!) I would think it's a better idea to use too much oil than not enough. The risk of the former is a fouled plug and particularly stinky exhaust (either of which is easily enough diagnosed and corrected the next time you mix gas). The risk of the latter is engine damage. The choice is yours, but the answer is, I think, obvious.
You can use synthetic or ordinary two-stroke oil. Both will be fine in your engine. Gasoline octane doesn't matter, either. The high-octane fuel will not harm your engine (although it will make a dent in your wallet over time), but won't provide any benefit.
I don't know your moped, but if it's a single speed than it's not surprising that it would rev up and sound like it needs another gear. It does! But multi-speed transmissions cost money and add complexity, and old mopeds were designed to minimize cost and maximize reliability.