thats a good point, as oxidization could... become a issue, is one of the only bi products of burning hydrogen is mostly hot water, then again hot water and stean become a cleaning agent. unless you count some of the motor oils that get into the combustion chamber. and as long as your piston rings and valve stems and other parts are good, the engine could stay very clean for a very long time!

hydrogen burns very thero, however it gets used very quickly beacuse its already in its natural gas state, while Gasoline and Diesel fuel are in a liquid state, and are vaporized, or carbonated. i think in this case they would get used much slower. but im not sure, it all depends.