A car's engine lubrication system is a complex piece of engineering that must be maintained to ensure a long service life. While it is widely known that low oil is bad for the engine, many people do not realize that too much engine oil can also cause a chain reaction of engine problems because the pressurized lubrication system is designed to work within certain thresholds. Too much engine oil can throw your engine's oil pressure out of whack, which can directly lead to secondary problems related to the pressure itself, and other issues resulting from the height of the oil in the crankcase.
Too much engine oil means that the oil level in the crankcase will be elevated, potentially causing the oil to come into contact with the crankshaft. The crankshaft spins at a few thousand revolutions per minute, which can cause the oil to become frothy and have a foam-like appearance as opposed to the rich and thick look and feel of normal oil. The aerated oil's degraded composition can cause the oil to overheat because its molecular structure has deviated form its original blueprint. Additionally, the foamy oil can lead to oxidation on the very surfaces that the oil was designed to protect.
Seals and Gaskets
Oil sitting too high in the crankcase can result in the oil coming into contact with seals and gaskets it would normally not encounter. These rubber components were not designed to come into contract with the engine oil and may degrade from interacting with the lubricant. The degradation of these seals and gaskets can come in multiple forms, whether it be expanding or shrinking, but the oil-induced deterioration of these components can result in repair bills and a decrease in the life of your car's engine.
Oil frothiness has secondary negative effects on the powertrain beyond the lack of lubrication that comes from frothiness. The foamy oil, which no longer possesses the manufacturer's intended viscosity, can clog up the oil pump. This can decrease the efficiency of the oil pump even after the foamy oil if flushed from the lubrication system. Prolonged or repeated exposure to frothy oil can lead to a complete shutdown of the oil pump, causing a costly and easily preventable repair.
The high level of oil can sometimes cause the engine to burn the excess just like gasoline. This can cause an unpleasant exhaust smell and blue smoke emission from the tailpipe. Burning oil can hurt your fuel efficiency and harm your exhaust system.