Indeed it is harmful to the engine. My cousin over filled his L200 back in '96, without checking the level, he drove his truck, he heard gurgling noises, the engine sputtering then without warning, the truck revved so hard, thick, I mean really thick white smoke bellowed from the exhaust and the worse thing is the engine cannot be turned off! the traffic behind him cannot move on or overtake him because the smoke totally obscured their vision. It took about 3 minutes or so before a concerned motorist approached him and opened the air filter to choke the air intake with a cloth. The engine stopped but the damage has been done, piston rings and all. After the overhaul, he sold the pick-up.