The reservoir should be filled to the top of the container using a wiper fluid solution which can be found in most automotive stores. Do not further dilute the mixture (unless manufacturers instructions tell you to do so) as this will affect it's ability to keep from freezing in low temperatures. Never place other fluids in the reservoir, such as a water/antifreeze mix, as other fluids could damage the pump seals.
Check the level of the windshield washer solution in the translucent reservoir tank at the same time the oil is being checked. Add the windshield water solution as needed.