If you are gonna work this engine hard at all or want it to last a long time, I would recommend a more robust oil than regualar M1 10W-30. At least use the M1 10W-30 High Mileage version as you will get a thicker 30 grade and higher antiwear additive level that these hard-working little air-cooled engines need.
Regular passenger car oils, conventional or synthetic, will probably work for a long time under normal conditions, but I like the added protection of one of the heavier duty oils mentioned so far. One more to consider is Castrol Syntec 0W-30 with the "European" on the label. This is the popular GC or german castrol(made in Germany)that folks here talk about. It is a very good choice as well and can usually be found at AutoZone or Pep boys.