Main Question or Discussion Point
It is said that if you overfill the engine oil. It can damage the engine. Why is that and is there a way to test if there is already damage?
You are conflating two distinctly different situations, so the catastrophe is not as bad as you make it out to be. The difference in the situations is in the path by which the non-standard fuel reaches the combustion chamber.If the engine is serviced at correct intervals this will correct the rising oil level, but if left, it is possible for the Diesel-rich oil to get past the rings, where it provides fuel for the engine, which will then run away uncontrollably. In this situation it cannot be stopped and will ultimately be destroyed.