I would be using an oil that has high temperature properties that exceed water-cooled motor requirements. Simply put, air cooled motors have less consistency and will have to deal with higher temperatures over the operating envelope. A modern multigrade oil with upper viscosity rating of ~40 or even ~50 would be my choice.
Mobil1 0W-40, a 15W-40, or 20W-50 would all seem to be appropriate, the choice based on both the cold weather requirements and expected warm weather upper limits.
I'm sure people have used inexpensive 10W-30's in these motors, but in my mind that is short of ideal.