It is best to monitor the oil level in your car as a matter of routine. But when a car is low on oil, there will usually be some symptoms that signal trouble. The oil light may flicker indicating the flow of oil through the engine has been reduced. Do not make the mistake of thinking things are okay because the light does not stay on. Flickering is a bad sign. Another sign of low oil is knocking in the engine. If either of these occur, you should check the oil and schedule an oil change as soon as possible.