i'd start with something safe like 32:1 (one ounce of regular 2-stroke oil per quart of gasoline). If there's lots of smoke or if oil drips from the tail pipe, cut back to 40:1 or whatever .. If you open the engine and see a nice coat of oil under the piston it's ok.
Lots of brrand new bikes around the late 1970's specify 25:1 but since that time oil formulations have improved .. my bike (PA50) smokes and drips bad at 25:1 but at 32:1 it's ok. Engine wear also has something to do with how much oil should be used. Adjust as necessary. Too much oil is a lot better than toasting the engine.. so use enough.
Personally i dont like synthetics.. they raise the cost of a gallon of gas about 50% or more.. and they dont lube any better than regular oils...
and they dont mix as well with gasoline because solvents are removed. The main thrust behind synthetics is the elimination of solvents in the oil.. it's a matter of reducing air pollution and 2-stroke emmisions and all that.. environmentalism..
and desperation by 2-stroke engine makers. Gotta find a product that sells under current laws as used in new vehicles to pass smog regulations..
and the outrageous advertizing and marketing claims for these oils is as slick as the oil, imho.