Placing paper towels around the filter port catches the little bit of oil that is lost when turning the filter quickly upside down to screw it back on. All in all, pretty painless. And, it saves more than 3 times its cost to have the service done by the company that supports Kohler in our area.
https://www.amazon.com/Kohler-GM62347-Maintenance-Residential-Generators/dp/B0088XMR84