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Lean burn at full throttle, like leaner than 12:1, can damage the engine.
But most engines run at a stoichiometric AFR when not warming up and when not being floored. Stoichiometric is the hottest AFR. I don't care what others believe, it is the point at when you are buring the most fuel per volume of air, therefore it is the hottest.
Leaning out the AFR leaner than stoichiometric actually causes the burn to be cooler. The leaner it is from there, the cooler it becomes. If you don't believe, the ones who have expirimented with this the most and have found this out are personal aircraft pilots. Running leaner caused their engines to run cooler and last longer. This was also my experience with my air cooled VW Beetle. I leaned out the low load jets so it would run at a 17:1 while cruising around, and then go up to a 12:1 when floored. The result was I had a hard time getting a warm engine, engine oil never would get to even 180°F and I got pretty good fuel mileage too.
Think about it, the extra air is innert because it doesn't have any fuel to react with. This is the same as EGR, inert gas that doesn't cause more heat. This is also why diesel engines run cooler inspite of running much leaner than their gasoline equivalents.
The main reason we've moved away from lean burn is emissions. Catalytic converters do a better job if they have very little oxygen. That way they can reduce NOx emissions, which need a reduced (low oxygen) environment in order to get the oxygen to separate from the nitrogen. This also why it is harder to reduce NOx emissions in diesel engines.
How lean, and when can make the ol' saying "lean is hot" either completely true or completely false. It's kind of like saying that rice is a high calorie food, then the go make those rice cakes that are sold as low calorie snacks. Go figure!
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