Oil surging to more than $120 a barrel last spring cut about three-tenths of a percentage point from U.S. economic output, but the blow was a small fraction of what would have taken place from a similar oil shock in the 1980s when the country was more reliant on imports, a new Dallas Fed study estimated. The roughly 15% cut to global oil supplies after the U.S.-backed war with Iran led to the closure of the Strait of Hormuz shipping lanes upended world commodity markets as prices rose, supplies became scarce in parts of the world and overall demand dropped.