This is most likely caused by a wear down of grease in certain areas of the door. Listen to it carefully, and try to ascertain the location of the squeak(s). Once you have determined the spot, you should lubricate it with grease, not oil. Contact a dealer or gas station to find out what type of grease your car will need, I'm sure it varies from car to car, but it should be something you can pick up at a gas station, I've seen hinge grease at hardware stores too.