Re Maintenance. During FEMA declared emergencies we make our contactor perform preventive maintenance (filter and fluid change) every 250 hrs of run time on diesel generators. They range in size from 20kw to 2Mw.
Also if you run your generator once a month/quarter/year etc as part of your maintenance program make sure it has a load. Running with no load just runs the engine and does not exercise the generator portion. Which will eventually cause problems.
Yes... periodic testing is important, especially if your genset is liquid-fueled (gasoline or diesel). One of the nice things about the one I bought is that it runs itself once per-week, under load, then shuts itself off. You can set the date/time/etc on the little controller panel inside the enclosure, or through software (the generator has an ethernet connection, and you can actually manage it, and see how it's doing remotely, in real-time)
Kohler remote generator monitoring is called OnCue, and requires a yearly subscription. I think it can also attach to your home network by wifi, with an optional wifi dongle.
It's actually pretty slick.