I have Mobil 1 synthetic 0w-40 in mine. For the Texas summer I should probably change to 15w-50. Walmart's here carry both.
Only Kymco Branded oil is going to have those specific 5w-50 numbers. The lower number helps it flow easily when cold, the higher number helps it perform once hot. 10w-30 would work fine if you don't ride when it's really hot or cold. It was riding around in 114 degree heat last year - and the stickers melted/peeled off of my bike (it was over 114 on the blacktop). I was glad I had 15w-50 in it then...
Just don't bother with the motorcycle version - it's differences are only for a wet clutch which you don't have.