I change the oil and use full synthetic after every renaissance faire run where the generator is used constantly. During the busy use, I try to change the oil as per the manual. Since the oil sees a lot more "use" by a splash lubricated generator, I use full synthetic.
After the season is over and the oil is changed, I just let it sit. It isn't going anywhere, nor is it oxidizing with the air in the pan, so I don't worry about it until the next year. I'm far more concerned about changing the gasoline out (it gets the StarTron treatment) because bad gas is the first thing that will kill a generator dead.
