for use AFTER the engine has been running. You are meant to check your oil within a couple of minutes of engine shut down. The dipstick measures the maximum amount of oil you can have in the sump without covering the turbo drains (in a tt) or the crank splashing it. A certain amount of oil still remains in the hoses and galleries (because you had it running a few minutes ago) so the oil level in the sump will only be slightly lower when the motor is running thus supplying you with the maximum amount of oil available for cooling and lubricating.