If you put too much oil in your engine, the crankshaft will reach the oil level. When this happens, the crankshaft will aerate the oil through a whipping motion. The air then destroys a protective layer of oil on the engine bearings, which keeps them from locking up. Once that protective layer is gone, the bearings will rub against other engine parts, causing the bearings to become shredded. As the bearings shred, bits of metal are flung about and will destroy your engine.