Detergent oils are meant to churn up particulates in the environment and trap them in an oil filter. Moped transmissions don’t have oil filters, so the dirt and particulates just stay suspended in transmission fluid all the time and lead to increased wear. Non-detergent oils allow the contaminants to settle to the bottom where it doesn’t hurt anything.
If you really want to stick to the letter of the law, people who sell lawn and garden supplies sell non-detergent oil for tractors. If you live in a city where there are no tractor supply stores, buy it on the internet, or just run detergent oil -- it isn't that big of a deal. Unless you have a Garelli, in which case the rubber clutches swell and get slick