In the old days with old oils they wanted you to mix at 20 to 1..you don't have to do anymore.
With modern 2-stroke oils you should probably pre-mix between 3 oz.( 43 to 1) or 4 oz.(32 to 1) of oil per gallon of gas.....and mopeds are designed to run on the lowest octane of gas you can buy (87 oct.) because they are afraid that somebody who doesn't know better will put the cheap stuff in..... and higher octane will not make it run better or faster or make more power.
And you don't have to buy the expensive synthetic oils..I run racing 2-strokes on the regular 2-stroke oils...with no carbon build up...it costs me $2.00/quart.