If you already have an Amsoil connection, why not run their 10W-30/SAE30 commercial oil that is made for those engines (and not too expensive)? You could run any number of their oils and do well. Their 10W-40, 0W-40, 5W-40, 15W-40, etc.
If you have a Schaeffer connection, then get some of their 15W-40 7000 blend or 5W-40 9000. Either of those would give you great service and long engine life in that harsh air-cooled environment, and for a great price.
Go at least 100 hours on any of these oils.