Why does this happen to this sawmill engine?

What happens here

This video demonstrates a simple and quick DIY method to check if gasoline has contaminated the motor oil in an engine. The process involves visually inspecting the oil for thinness and then performing a flammability test by comparing a drop of the suspected engine oil with a drop of fresh motor oil on a metal surface using a flame. If the engine oil ignites, it confirms the presence of gasoline.