Overfilling your oil tank results in less effective lubrication that may result in damage to your vehicle's engine. According to Car Talk, too much oil can make it difficult to properly lubricate key engine parts.
Oil is essential in lubricating the moving parts of an engine. Overfilling a tank can place greater strain on seals and gaskets thereby increasing the likelihood of developing an oil leak. Oil that has come into direct contact with the crankshaft may be whipped into a froth that is too thick for the oil pump to manage effectively, reducing the lubrication needed to protect key engine parts.