If you are running your average spamcan 4cylinder rental at cruise at75% power or less how can you "overlean"
It's near impossible.
Cracked cylinders are generally the result of throttle abuse, not mixture control abuse. For those aircraft equipped with a cylinder head temperature indicator, watch how the temperature cools as you carefully reduce power in cool air. Then think what happens after a hard climb, and the throttle is suddenly closed.... Shock cooling equals cracked cylinders.
Generally, and aircraft with cracked cylinders will produce somewhat less power, but will continue to run safely, so is not a safety concern for the next pilot, but certainly a cost concern for the aircraft owner.