Too much oil in an engine can damage it. Adding too much oil to an engine can be as bad as running out of oil. The engine needs to be filled with the factory-standard level of oil, since either a low or a high level can be disastrous.
Too much oil in the engine increases inner pressure that can lead to oil leaks around gaskets and seals. The excess oil can also clog the catalytic converter, fool the oxygen sensor and ruin piston rings. Too much oil in the crankshaft pan eventually causes the oil to froth and lose its properties. The frothy oil cannot lubricate the engine components properly, leading to a faulty engine.