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You need a CERN Driving License (go to EDH, click "Other Tasks" and "Car Driving Authorisation") to use it, and a Mission Order (go to EDH, and click Other Tasks and then Mission Order, needs CERN Driving License to be approved) for offsite use such as going to the apartments. According to Conditions of Use drivers need to have a bunch papers with them while driving a CERN car: national driving license, passport or ID, CERN attestation, Mission order (if going outside CERN perimeter).
As of March 2010, there is a new way for filling up. Go to the fuel pumps on the west side of Building 130. There is a badge attached to the car key. Hold it next to the icon on the pump. It should display the CERN Car number (3953), and then ask what fuel you want. Choose unleaded using the corresponding number code. Enter the mileage. Now you can fill up. This facility is supposed to be operational 24/7.