The oil might touch something it is not supposed to. It may foam, froth, aerasolize. The aerosol oil mixture can clog up the PVC valve, too much oil may escape the PVC valve. If the engine is the type that burns the PVC exhaust (to reduce smog) then you will be burning oil, the burned oil, especially the metal in the oil can coat and damage the catalytic converter and oxygen sensors, you can foul the spark plugs. In short, a lot of really bad things can happen if you overfill. It could be no damage but reduced mileage. How overfilled is it? Is it still within the dipstick marks (within acceptable range). Or is it way overfilled. If it is still within acceptable range, then nothing bad I presume. Also the engine have some tolerance. So if you were to drive up a steep incline, the engine would not destroy itself. So even if you overfill, if you are within this tolerance, you may be OK.
Open the hood and check the oil level. No need to guess. Bad mileage and possible engine damage should be the results of an overfill.
1. If it's just a litre or quart, nothing. It -may- decrease your mileage a bit but that's it. 2. If it's more than about two litres or quarts then it -could- overflow into the intake. 3. If it's more than about 3 litres or quarts then it -could- cause mechanical damage. As stated above. Check the dip stick. If it's only 1/8 to 1/4 inch above full, no problem as in 1. I would suspect this would be all anyone would see. If it's way above that, then have them correct asap. Make sure the manager hears about it. It's unacceptable if they add that much.
