Nitro content in fuel varies, normally in the range 16% (ready to run) - 30%.
In a race engine (Bullit, Alpha, OS, RB etc.) you'd normally go for 25% or 30%. 30% is slightly easier to get a good idle on I find, and offers a performance advantage, however all but the best drivers won't notice! 30% is pretty much being phased out too. If you do go for 30 you may have to re-shim your engine head to alter the compression, the motor will most likely come with the shims and guidance on this.
As for brands, I always used Byrons as it was easily available, but Piranha, Firepower or any of the others will do just as well. Pick a brand and %age and stick with it. You can change brand if needed but would have to retune the engine so it's easier to just stick with a brand. For my money, 25% Byrons Gen2 is as good a place to be as any.
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