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 v5.13.1
Source: github repository
Note: Module Manager is required.
From the
From a long discussion with NathanKell: How to make new tanks and/or make parts use MFT tanks First you need a TANK_DEFINITION, which has the following fields: name = the name of the TANK_DEFINITION. basemass = modifies the dry mass of the part. Use a value to replace the part's mass, like 0.01. Add " * volume" after the value to make it depend on the tank volume, like "0.0002 * volume". Or use -1 to leave the part's mass alone. One or more TANK blocks. TANK has the following fields: name