James is correct, however it is not always necessary to add oil to the system especially if it's a slow leak. Too much oil in the system is not a good thing. Secondly most new cars have both a low and high pressure switch. The low pressure switch if the system is low on charge and high pressure pressure activates the condensor/ radiator fan or fans.
You really should attempt to find the leak and have it repaired, if moisture gets in the system it forms an acid and eats everything from the inside out !

Good Luck