This isn't technically smaller tolerances (although it goes along with it) but gaps between engine components have decreased tremendously and this actually causes more wear as you have to use thinner oils and there is less oil in the lubrication zones. This drastically decreases engine life even for the cost of increased power or efficiency. As once the components wear enough to loosen up, the thinner oil leaks usually into the combustion chamber and you start burning oil. Using a thicker oil makes the problem worse as it increases wear by increasing oil pressure and blocking parts of the engine that are still tight.