Possible positive side effects.
I assume hydrogen will not act like a solvent like gasoline. I will not tend to "wash" cylinder walls of lubrication and break down motor oil. This may mean longer lasting engines and a much greater oil change interval leading to less waste oil to deal with.
Something I have wondered about. In the combustion of fossil fuel, for every gallon you burn, you create almost a gallon of water. (Hence Lindy's contrails)
I couldn't find it but I'm wondering what is the quantity of oil we have taken out of the ground and returned to the planet surface as water and if this has changed our environment at all?