It depends on your definition of 'half full' as if the oil is halfway between the 'minimum' and 'maximum' point on the dipstick it is not half-full. Even just above the minimum oil point there is a safe amount of oil to run the engine with. It's when the dipstick is completely dry or below this minimum that you need to be concerned. Filling oil to its maximum point risks over-oiling if done without care, which can be similarly damaging.