The previous Modular Fuel System has split into Modular Fuel Tanks (taken care of by myself) and Real Fuels (taken care of by NathanKell).
Modular Fuel Tanks allows any supported tank to be filled with exactly how much or how little fuel you want, of whatever type you want (though different tanks may allow or disallow certain fuels; jet fuel tanks won't take oxidizer for instance).
Download: MFT v5.12.3
Source: github repository
Note: Module Manager is required.