When changing your own oil in your car or having it done by someone else, you should always check the oil level when done. Many worry only about having too little oil in their cars. However, overfilling the oil in your car can cause as much damage as under-filling it.
The crankshaft rotates at a high rate. Too much oil in the engine can cause the crankshaft to whip the excess oil to a foamy froth. This causes the oil to heat up and oxidise. The now spongy oil becomes harder to pump, causing the oil pressure to drop. Less oil pressure means parts of the engine can become starved for lack of proper lubrication. If left untreated the engine can eventually lock up, causing major damage.
Overfilled oil expands and puts pressure on the oil filter gasket. Over time, the gasket can either crack or separate from the pressure. Once this happens oil will begin leaking out of the engine, reducing oil pressure in the engine as well. As noted above, reduced oil pressure causes lubrication problems for the entire engine.
Reduced Fuel Economy
Too much oil causes the crankshaft to work harder pushing the extra oil. This reduces power in the car. The driver then uses more gas to make up for the loss of power to match the speed desired. Reduced fuel economy results from this action.
Check the oil level in your car after each oil change or if you notice smoke billowing out from under the bonnet, reduced power as you drive or oil leakage. If the level on your dipstick rests above the top level either drain the oil yourself by unscrewing the drain plug at the base of the oil pan or take your car to a service station to have the oil drained.