The
oil stays in the cooler, if it drained out then there would be a dry start every time you fired up the motor. You could drain the cooler and then prelube the motor before fire-up every time but who would. So you only change 5 out of the 6 quarts of
oil in there, big deal, close enough. You can never get all the dirty
oil out anyway, it still coats all the internals.