Once ownership is taken of a mini, another player cannot get into the pilot seat, this is expected. A player not friends, or team can still open the fuel tank and remove lowgrade.
Jack & I tested this with a mini I called in. There was a random player there, so we had them test for us. They were able to open the fuel tank and remove fuel. When we ran looters on the fuel tank, there were no logs.
Server: Scourge