Tom-Guy

4th and 5th gear pulls take longer, so heat has more time to build up in the engine.

If you're making enough power, 4th and 5th gear pulls don't take so long, so they aren't quite as bad on the engine.

Realistically, unburnt gasoline is not what is cooling the engine, water is. For every pound of fuel injected approx two pounds of water is formed. A richer mixture tends to form more water, but there is a cap to how much is formed based on air mass (oxygen) ingested regardless of how rich the mixture is. Fuels like alcohol form water earlier in the combustion reaction + are why alcohol engines run cooler and some of the track-only alky rides forgo a cooling system + just totally fill their blocks. There is a reason why I advocate water injection, or fuels that contain less hydrogen and more nitrogen (higher power combustion rxn that also has more "octane").