I use synthetic oil, too, in my scoot, but I believe the full benefit of synthetic is kind of lost in a scoot with such short oil change intervals and no oil filter.
A big benefit of synthetics is they hold up better for longer intervals...but our scoots need frequent changes.
Any modern oil that meets the specs will work fine...after that it comes down to picking what you like, can easily find and doesn't cost too much $.
(remember, these motors are not like motorcycle engines, which share oil between engine and transmission...they are more like lawnmowers!)