Welcome to the forum! Personally, I don't like multi-viscosity oils in the old two cylinders. It would probably be fine, but I prefer straight sae 30 for summer use. You dont mention where you are from and temperature ranges you are looking at, but for any temp above freezing, the sae 30 would be fine. The tolerances weren't as tight when those engines were built. That is just my opinion, I am sure you will get other ideas as well.
These machines were engineered to have dirty pond water in the radiator for coolant, or so it seems.:laugh: The tolerances are not what they are today in modern machines. They are so much looser than today that just about any quality oil will take car of the engine just fine. I personally will run SAE 30 in my Johnny Popper (like jd110 suggested) when she's back up and running. If I couldn't find that, I'd use a thicker multi-viscosity oil such as 20W50. :good2: