Cujet post here on BITOG #1780297 - 02/13/10 11:06 AM
would be a good read. Cujet has several post here on BITOG about how good Mobil 15W50 is and in some of those post he wrote about neighbors air cooled engines not surviving in the Florida heat while his continue to run without problem with the 15W50.
There are many good oils. GC, Rotella T6 5W40, M1 TDT, Mobil 0W40, Mobil 5W40, Mobil 15W50, Mobil V Twin 20W50 The thicker oils (50 weights) would be for HOT days.
Personalty I would stay away from Rotella T5 if it is still not JASOMA rated. Rotella T6 is JASOMA rated.
I keep GC in my two gen-sets, 5000 / 6250 Watt with Tecumseh 10 HP, and Generac inverter type 2000 watt.
However I also keep two quarts of Mobil V Twin 20W50, and two quarts of Mobil 15W50 for hot days.
There use to be an oil for air cooled engines that run on natural gas. I think RedLine made it, but I looked on their site and do not see it.