I’ll disagree about oil. The Best fuel for a backup generator (I have a 36 kw unit, so I’m speaking from direct experience) is propane,which doesn’t contain additives and doesn’t break down over long storage the way diesel fuel can.
Plus the nice thing about a propane generator is that they cycle on once a week for 15 minutes to keep everything working properly. Then if you lose power they turn on almost immediately.
My daughter/son inlaw have one at their house. I have a Honda 7500 watt gasoline generator. It is wired through a transfer switch and panel that runs pretty much everything except the clothes dryer.
I’ll disagree with both of you all. Best fuel for a generator is natural gas. No storage issues.