i've run 15W-40 in my Honda gennies for days at a time. the last time was for 18 days straight with 50 hour oil changes. i'm sure that i could have run them to 100 intervals but i had excess oil on hand for the eighteen days. i also ran some SAE 30 straight viscosity oil as well.
the big Honda powered Winco would run about 10-12 hours a night. the Honda EU2000i would run from 6am to 6pm average. so a 12 hour day. then the Winco would take over from 6pm to 6am average. the Winco ran the AC and everything in the house and consume 7 gallons a night.
my moms EU2000i was running for 24/7. only turned off on oil changes. i would even refuel while running. lol
the 15W-40 and the monograde 30 oil protected my air cooled equipment, nicely.
for light duty use i would def run 10W-30 if that the only oil i had on hand. for hard use where i was running my gens at max load, during an emergency, i would only trust 15W40 or straight 30 or 40.
*had neighbors gens breakdown during the 18 day power outtage due to using 10W-30 and 5W-30 oils. and lack of checking the oil levels.*