Honda GX390 13 hp air cooled, Blackmax Generator 8kw , my dad bought it new for Hurricane Katrina, so its knocking on 15 years old but it has never given him a lick of trouble and doesnt have many hours ( thankfully, low hours means not much hurricane activity). I've maintained it for him using various 30 and 40 wt oils, but I'm considering switching to either M1 15W50, as recommended by Cujet, or a natural gas specific 15W40 oil. I brought up the natural gas oil due to the fact that my dad had a natural gas conversion kit put on it years ago, he has a hose under the house with a valve that he connects to it and can run it continuously until either the power comes back on or when he has to stop it to change the oil, which I do every 50 hours. So should I go with the 15W50, or the natural gas 15W40, and should I keep the oci at 50 hours or extend it to 100? I'm also considering the fact that the 15W40 NG oil is normally used in liquid cooled NG engines, so it may not be the best oil for this air cooled, high heat engine.