IIRC, the game automatically burns off the AUX tank before the wing tanks. Think the AUX only gets juice if you go over 50%. Your best option is to take 50% and DTs and drop one DT on take off. Then you can cruise or lighten your load and engage at your discretion.
To shift fuel tanks try SHIFT-F - that cycles between auto-select (white txt on gauge), and individual tanks (yellow on gauge). So it will cycle between Auto, AUX, Left and Right wing tanks, then Auto again etc. The auto-select will burn AUX first then alternate sips from each wing tank in turn to keep you balanced. If you switch to manual tank select, just remember to turn it back to auto or be ready to do lots of manual switching to keep wing tanks balanced. Also on manual select, the gauge only shows the fuel remaining in the selected tank.
Combat flaps will deploy at crazily high speeds - far too high IMO. IIRC, its over 350?? Dropping flaps at those speed wont help as you are limited by Gs anyway and only increase drag, which is not good given the poor acceleration qualities of the pony. Drop flaps when you come off the black-outs and start hearing stall buzzer. Flaps are useful for getting nose around/over/pulled back up when your getting too slow. Pull em back in as soon as you no longer need em. They induce drag and cost E.
As already mention the pony turns wickedly well at high speeds, more than a match for anything else in the air. The only prob is it doesn't have enough power to replace the lost speed from turning too hard, so this limits you to a few hard turns before you have to re-assess your next move. Keep her around 270 (+-10) and she'll treat you well. Set convergence in close and go hunting for the snapshots! Get out if it ain't going your way, as sustained turn contests is not its cup of tea. Make sure you leave your self enough speed or alt to be able to escape when required. Until you reach your failsafe point, push it hard and go hunting for them snapshots. The trick is to know when to cut your losses. Most wont expect you to be so aggressive in it, and you can often get them on the back foot pretty quickly, but you have to know if things are going your way in the first few turns or not.
And while the B pony can turn ever so slightly better than the D, the B lack of deck speed, only 4x50cals, and its natural high-altitude ability means its not the best option for a low furball turn-fest. The D is a much better low-alt option, IMO. Up high (above 12k) the B comes into its own and is a beast!
Here's a write up i did a while back:
http://www.my2cents.co.nz/AKUAG/TheVault_Files/P51Guide.aspx