With high oil levels the crankshaft cuts through the oil more frequently and creates oil mist. If the engine oil is unsuitable, contaminated or degraded (old) then this cutting can result in foaming. This foaming combined with the blow-by gasses will allow the mist to enter the intake through the crankcase ventilation. Once it enters the intake, it is sucked into the combustion chamber and burnt off. This will also be visible on spark plug tips being black rather than light brown.