The typical combustion process, whether gasoline or diesel, consists of breaking down hydrocarbons in the fuel and recombining with oxygen to produce carbon dioxide and water vapor. This is considered an ideal combustion.
However, engine wear, impure fuel, and design imperfections mean an ideal reaction does not occur. In this incomplete combustion, carbon is left over and secondary pollutants are produced as harmful emissions while the carbon builds up in the engine and in the exhaust. black stuff you see in exhaust stacks and inside an engine. In refineries it is also called coke. This carbon build up is the number one killer of engines.
Therefore with less fuel being burned for the same amount of energy and less harmful byproducts occurring in the engine one would expect to see an extended engine life with less repair needed.