If too much oil is put into a car, it starts to leak when the engine runs. This leakage is due to excess pressure generated by the oil pump and crankcase placing strain on the seal on the rear main bearing.
If the excess pressure on the seal is not relieved, the seal ruptures, increasing the oil that leaks out. Once the oil begins leaking from a ruptured seal, the oil passes into the clutch and causes more damage to the vehicle. If the problem is still not corrected, the front seal bursts and causes the engine to gush oil. In addition to drowning the clutch in oil, which causes the clutch to slip and malfunction, the excess oil coming through the failed front seal leaks into the engine bay and can be hurled out as far as the front disc brakes. Overfilling the oil also causes the rotating crank to churn the excess oil into a froth. This aerated oil is pushed into the bearings and afterward into the pistons where the piston rings begin to break down from trying to deal with the excess oil and pressure. Excess oil can also work its way into the combustion system and out through the exhaust. By this point the excess oil has potentially caused irreversible damage to the engine.