After I broke my Honda powered generator in with petroleum oil I have been running Mobil! 15/40 in it. Bought it in 1996 and it still runs every time I race. It sometimes runs for days at a time and it gets hot in SC. RM
I have 3400 hours on my Coleman 6500W generator. I change it every month with Havoline full synthetic and clean the air filter out. 5w30 in the colder months, 10w30 the rest of the time. Pulled the valve cover off a year ago, everything looked brand new. Still has the original plug.
Honda generator oil isn't much more than good quality SAE30 motor oil.