What is the problem if the temperature of the oil and heads of cylinders is too high?
Piston engines are designed to run within specific temperature ranges. Going above this range might effect the lubrication [hot oil may not provide the right amount of protection to moving parts, If hot enough it could melt seals, or even catch fire.]
the other danger is as metal changes temperature its structural property's change. If a metal gets too hot it becomes softer and may become less resistance to forces. also there is the issue of shock cooling, if a hot engine gets cooled to quickly, it may unevenly cool, and due to the difference in heat expansion could crack or fail.