To tell the truth, on my first bike, I've never changed it, and the previous owner
never did either. 30.000km and no issues on that bike (50cc peugeot scoot).
I did change it on my Agility 125.. I think twice in 20.000km..
Just use any oil that matches the viscosity stated in the owner's manual.
I grab any "good brand" 80W90.