Additives are very common and well regulated.
The second part of the question is more tricky because it comes to thermodynamics - what amount of energy will be released from the fuel. No matter what is added to fuel it will have more-less amount of energy per mass (*unless we are talking about the mixtures with ethanol, but that's the different topic). The only way how fuel consumption could be decreased would be to provide more consistent, more stable mixture of air and fuel. In practice, it could be seen only in extreme cases: fantastic gasoline vs the worst possible gasoline, or if the engine is a tech. masterpiece that runs well only with the best fuel.