Originally Posted by Gillis
Can you explain more about this please?
Sure! If the oil level is high enough to allow the crankshaft counter-weights to "beat" into the oil in your pan, it foams up a bit from air being "whipped" into the oil.
Valve lifters are small hydraulic pumps, and depend on air-free oil to operate properly. By introducing aerated oil to them, it can affect performance and create undue lifter tick. Air compresses; oil does not. That small bit of compression caused by aerated oil can cost a few thousandths of an inch of valve lift, thus affecting overall performance of the engine.
In extremely aerated oil (filling way beyond the full mark), you can end up with cylinder scoring and bearing damage. Rare, but it could happen.