Leaning is not only for better mileage. The engines usually do not like a fat mixture for prolonged periods of time... I can’t recall all the reasons, but I think a couple of mayor ones were: 1) lead in the avgas will foul the spark plugs for rich mixture settings which is a safety risk, 2) exhaust gas temperatures will be lower which can lead to a build-up at the exhaust valves which can lead to wear and failure and 3) you end up with more dirt in your oil which makes engine corrosion even a bigger possibility...
Lean if you love your engine. And read Mike Busch - because he has the facts better than I do...