i'm not a certified advice giver in this area-
but, my gut tells me- from the information I have accumulated through this forum and other sources, that an air cooled engine is going to be prone to less controlled engine temps, sometimes higher than a liquid cooled design. Which would definetally steer me to recomend, a synthetic, that meets diesel certifications, of the highest weight rating that it can safely pump at good flow.. though, all diesel oils I have seen are 40 weight. (with varying "W" ratings usually corresponding to whether or not they are synthetic or not). Synthetics, in heavy weights, won't shear from heat or burn off as easy as a regular dino oil.
so basically, i suggest a synthetic, delvac 1 is the first that comes to mind at the moment, but i'm sure there are other options that are equally good.