well, when you have that much oil in there it churns and makes a foamy bubbly substance in there and it can really cause problems.
the same goes for automatic transmission fluid. 2 quarts overfill is not a good thing. Do a nice oil change correctly, then run a compression test.
remove all the plugs, prop the gas pedal to wide open throttle and screw the tester into a cylinder and crank the engine over 5 or 6 times.
do each cylinder and they should be within 10% of each other.