The previous Modular Fuel System has split into Modular Fuel Tanks (taken care of by taniwha; for all your stock-resource modular-tank needs) and Real Fuels (by me, modular tanks and engines using real resources). Use one OR the other.
Contributors: NathanKell, Chestburster, Starwaster, taniwha, swamp_ig, regex, ialdabaoth (obviously, but back again!), blowfish
ialdabaoth (who is awesome) created Modular Fuels, and this is a fork of the RealFuels branch.