What type of gear oil do you use in a fa50?

the manual says to use 500ml (.5qt) of 20-40, another person says 10-30 synthetic could improve top end

The FA50 should only have 10w30 oil. Whether normal or synthetic will not make any difference to the performance. Too much oil will cause problems so measure carefully. The gear train sits in the oil and so does the clutch. Replace the oil once a year at least. I often add a little oi additive like Slick50 or Motorcoat to keep the gears well lubricated.

use the lighter gear oil and the web sights says to use only 300ml of gear oil to add 3-5 mph on the scoot. they sd that 500ml is to much and causes drag in the gears. using 300ml will run great and protect as well as 500ml good luck

from 80/90 gear oil to 10/30 which one and how much?????

That depends on a few things, like altitude and the atmospheric temperature. Most times, anything on 2 wheels will do fine with a 80w90 gear oil. If you can find. You can find this at any automotive-supply store, or anywhere with a decent automotive department.