Welcome Kymco Owners!
After looking around, I could not find a 15-40 full synthetic oil. I am going with Mobil 1 10-40 for the motor, and Mobil 1 75-90 for the transmission. Will there be any problems mechanically or with the warranty with these choices?
If the manual states that you should use 15W40, there's obviously no need to go fully synthetic, and synthetic is not always better.Your bike will run fine, and the guys in the shop will not know you didn't use 15W40.p.s. Changing the oil by yourself will void warranty in a lot of places so check that first.
I have a quart of Rotella T 15W40. It meets API service CJ-4, CI-4 PLUS, CH-4, CG-4, CF-4, CF/SM and also meets the requirements JASO DH-2, and JASO MA which is for motorcycles. I used it in my Kymco People 150 without any issues.