Because water-cooled and air-cooled 2-strokes do not run at the same temperature and never run the same 2-stroke oil. This is ... well known.amsoil has a 2 stroke synthetic they say you can mix at 100:1 regardless of the mix ratio specified by the manufacturer https://www.amsoil.ca/p/saber-profe...K8yl3xUc0Yh98AfBbbmM1-6g-MUoaP1EaArujEALw_wcB
although they recomend it for chain saw applocations, which is how i came to have a bottle. not sure why you couldn't use it in an outboard.