I've got two of these engines. I'll be looking forward to the reply back from the OP, but remember that these engines were probably built in the 50s or 60s, run hot because they are air-cooled, have a tendency to vapor lock because the carb sits in a hot spot and have a tendency for the head bolts to seize to the head. They also need to have all air shields in place and the dirt must be blown out every day.
On the bright side, parts are relatively cheap if you are patient and get them through ebay sellers cleaning out their NOS stock and the fuel pumps can still be purchased through NAPA. (That's what the ebay sellers are doing. They sell a fuel pump at a rediculously jacked-up price and then order it at the local NAPA really cheap AFTER the sale is completed on ebay)
To the OP, if these engines are parade queens, any 30W will work. If they are run hard, use a good 15W-40 synthetic that still has some zinc in it. Remember that oil technology has come a long way since the operator/maintenance manuals initially came out.