I put Castrol 4 stroke bike oil into my YS125 (18 plate) when I had it. No issues using that in my experience.
Castrol 15043F Power 1 Engine Oil 10W-40 4T, 4L https://smile.amazon.co.uk/dp/B004WMDN92/ref=cm_sw_r_cp_api_glt_fabc_C4QWT1EEDJTFD6NX9M7J?_encoding=UTF8&psc=1
Might get it cheaper elsewhere - but 4 litres is enough to easily do 3 full oil changes with filter changes too on the YS125.
I also put together a video showing the oil change :
Hope it helps