What happens when you start to burn H2 instead of C8H18 (most common element in petrol). Well... you get a BIG BANG and water vapor (H20). H2 burns over 50 times faster than any ingredient in petrol. To burn one unit of C8H18, you need 21 units of O2. And you say, it doesn't use a lot of Oxygen... check your facts before you say anything. CO2 isn't harmful most cases. But put it together with H2O (water) and you get H2CO3, witch is acid. Pretty strong one actually. In Petrol there is also some S8 So it too burns and takes some O2's creating SO2, witch also reacts with water to make H2SO3, one of the lowest pH levels.
2C8H18*S8+41O2->16CO2+18H2O+16SO2
There are many safer and more energy efficient ways to make H2 IMHO. Take Alkali metals or Alkaline earth metals, add some water and BOOM you got some H2.
Or metal + acid -> salt + H2
etc.
you could also make some NO2. One of the byproducts of this is H2. But I'm not going to give the formula here. just in case.
--Rain--
PS Chemistry exam coming soon