When an engine has too little oil, bad things can happen: friction between moving parts increases, the engine may overheat, and in the worst-case scenario, moving parts seize due to lack of lubrication.
Bad things can also occur when there is too much oil. Overfilling with oil can cause foaming, which turns a slippery lubricant into a frothy liquid with air bubbles that reduce lubrication and cooling properties. Foamy oil is also harder for the oil pump to distribute throughout the engine, so some moving parts may not receive as much oil as they need, increasing wear and the risk of engine damage.
How much oil does a car need? It depends. Check the details in your vehicle’s owner’s manual, but many engines require between 4 and 6 quarts. Typically, oil flows into a pan at the bottom of the engine, below the crankshaft. If the pan is overfilled, the crankshaft can act like a high-speed mixer that churns the oil and creates foam.
The crankshaft is where engine speed is measured, and it rotates rapidly whenever the engine is running. For example, if the tachometer reads 2,500 rpm at 75 mph, that means the crankshaft makes a full rotation 2,500 times per minute; flooring the accelerator can bring it to 5,000 rpm or more.
Overfilling also increases oil pressure, putting more strain on the gaskets and seals that prevent oil from leaking out of the engine or going where it shouldn’t inside. Over time, the extra stress accelerates the wear on these gaskets and seals. Overfilling often happens because repair shops pump fresh oil from bulk containers instead of pouring it from quart or gallon containers. Holding the pump trigger too long can easily add extra oil, and the technician may not check the dipstick—and even if they do, oil takes time to drain into the pan, so they might get a false reading of the oil level after adding fresh oil.
Checking the dipstick when the vehicle is on level ground and the engine is cold will give an accurate reading of the level. If the oil level is slightly above the full mark, it shouldn’t cause problems. If it’s overfilled by half a quart or more, or if foam appears on the dipstick, the best solution is to drain the oil and refill it to the appropriate level.