Ok, I broke out the owner's manual. It says "Yamalube 4 (10W30) or SAE 10W30 type SE motor oil". And you will need about .3 L, so one bottle will cover 3 changes.
I apologize if I have caused any confusion, it has been 3 years since my son has even ridden his PW50, so my recollection is a little fuzzy. I though that Yamaha had a diferent case oil for the 2-strokes. I always used Motul 600 for the engine, it kept things a lot cleaner. I did dump the Yamalube in the case, but I trusted the parts guy I had back then, and never really paid attention to what it said on the bottle.