It's always a good idea to start with the manufacturers suggestions and work from there. I know what you're thinking, that a good HDEO will have more adds which might be beneficial to small air cooled engines, right. While there doesn't seem to be any harm in that thinking, a VOA of B&S'S oil shows no more adds than SL PCMO, so no real need for an over abundance. I would think that a good HM syn like Mobil 1 HM 5W30 would be near perfect. It has SL level zddp without going overboard. Then again, any of the xW30 syn oils would be excellent. One of the best woud be GC 0W30, or something along those lines. Remember, these types of engines have been run with almost any oil and survived, so add packs, while not crucial, aren't life and death.