Guzzijohn I think the syn is the right answer. Heat is the killer of motor oil, if that generator is air cooled you really have heat issues if its summer time, or the generator is parked out of the way where fresh air is harder to get at.
Secondly very few will argue that syn oil is of equal or lessor quality than mineral oils, and will outlast them in head to head comparisons. What is generally argued though is that you don't need extra strength protection. If the economics of running that generator are a concern I really don't think there would be a generator sitting there to talk about. It's sitting there to protect something, be it your sanity or whatever. What is another $4 a yr on oil?
Lastly I don't know where you live but in Mn. it gets plenty cold. Synthetics flow better in the cold, no arguement there. It's going to start and run easier in arctic conditions and high heat both. Do you get to schedule emergency use or maintenance of the generator for adverse weather?
Drawing moisture? Give me a break, its oil and additives, more pure and consistant than mineral oil sure, but it's still oil. Now if the generator sits and draws moisture and you NEVER change the synthetic yes you will see water problems.
Your mechanic probably sees engines where the owner is expecting to go twenty thousand miles between changes. Most riders take years to do that and that is where the water is coming from. Another place you get this is gear boxes that are lubed for life. No one wants to read the fine print that says if submerged you need to drop the lube. It is excellant oil but is not a substitute for a brain.
