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The idea that lean mixtures at high loads burns holes in pistons is correct, but it is for a different reason than some people think.

The exhaust gases are always hot, to different degrees. However, with a lean mixture, there is extra oxygen left over in the combustion products (by the very definition of lean).

Combining very hot oxygen with hot aluminum results in the aluminum (the piston) burning (the aluminum chemically combines with the left-over oxygen), just like if it had been hit with an oxy-acetylene gas cutting torch! The burning of the aluminum adds further heat, and the process gets catastrophically destructive in a hurry.

The chemical equation is:

4 Al + 3 O2 → 2 Al2O3

Note that at low loads there isn't enough heat in the combustion products to raise the piston temperature enough to initiate burning of the aluminum, so lean mixtures at low loads are safe (and save fuel).

With rich mixtures, the aluminum can't burn (there's no oxygen left after combustion, and it's too cool in the chamber before combustion), it has to melt instead, and this harder to do (the endothermic phase change absorbs a fair bit of energy, unlike the exothermic burning of a lean mix which gives off energy).

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