Everythign you leave on the map when finishing the mission, gets owned by the map. That is, if you do leave a vehicle that could´ve been repaired, and later returns to the map, you can repair the vehicle and use it on that map, but will lose it when the mission is done.
Regarding the fuel issue: Shermans, Stuarts, etc uses Petrol as fuel. Dorchester, Matildas, Churchills uses Diesel as fuel. That´s why you can´t refuel your Matilda from a Sherman etc.
A damaged tank turret (red colored) wont make you lose the vehicle, it will be automatically repaired in-between battles. However a totally greyed out turret means that you will lose the vehicle (this is due to the fact that the game engine doesnt allow us to man vehicles that has lost their turret but is otherwise undamaged. Too bad, the old S:HOWW2 engine allowed us to do that and it was pretty cool to drive around in a turretless tank!
).
~Zeke.