Can an extra quart of engine oil hurt my engine?
It's not ideal, but whether it will actually hurt or not depends
on the specific design of the engine. Some engine designs can
tolerate excess oil better than others. The most likely problem is
that the level of the oil will get too close to the moving parts in
the engine, which can result in air being "whipped" into the oil.
This is a bad thing for several reasons, not least because the air
can get sucked into the pump resulting in "dry" spots in the oil
lines.On many cars, the optimum oil level is marked on the dipstick
as a range rather than a specific line. A quart over the bottom of
the fill zone is almost certainly not a problem. A quart over the
top is more worrisome.