I usually fly the P-38 with the smaller ammo load, 50% fuel and a single drop tank, unless I'm defending, then just 50% is enough. With the lighter ammo load, performance is noticably improved, especially when tangling with fighters like Spitfires, Nikis and the Ki-84.
P-47Ds: 6 guns short clip, 50% and belly tank or, 8 guns same fuel. There's a trade-off here, better performance vs more punch. However, the performance difference isn't huge. Loading 8 guns and full ammo will make a significant difference though.
Rather than take a P-51D with 4 guns, I'll just take a P-51B. It handles better anyway. Yeah, the P-51D is a bit faster on the deck, but between 14k and 20k the P-51B may be the best fighter in the game. You can fly the F4F-4 with 4 guns, but I woundn't bother when the better performing FM-2 already has 4 guns. Likewise, when flying the P-40E I'll load 50% and the belly tank. That will take you about 4 sectors as fuel comsumption is excellent in the P-40. But, due to a small ammo load-out, the P-40 usually runs out of ammo before it runs out of gas.
As a rule, I NEVER take more fuel than I actually need. Simply loading external fuel will get you anywhere you may want to go.
My regards,
Widewing