use the straight 30 weight oil the 5w30 is 5 weight oil that can take 30 weight oil abuse. the 10w30 is 10 weight oil that can take 30 weight oil abuse. most air cooled engine take even 40 weight oil but oil makers are going away from 40 weight. my corvair takes 40 weight oil. anything lighter and it just evaporates away.
just so you know an air cooled engine runs way hotter that a water cooled evhing. what would you do if your car’s temp gage got to 325 degrees? my corvair is just right at that temp just to give you an idea of the operating temps of air cooled engines