Bug observed on US Scourge by a player.
When their teammate's minicopter was parked at their base where they both have building privilege, they were able to access their teammate's minicopter's fuel tank and add fuel and then fly it away, as expected.
However, once flown out of building privilege zone and landed, they were unable to access the fuel tank. They were still, however, able to fly the minicopter back to their base. I am fairly certain that teammates should be able to access other teammates' vehicles. Being unable to access the fuel tank (unless in their building privilege zone) can pose a potential problem.