Indian Oil Corp, the country’s top refiner, has signed its first annual deal to buy U.S. oil, paying about $1.5 billion for 60,000 barrels a day in the year to March 2020 to diversify its crude sources, its chairman said on Monday. IOC is the first Indian state refiner to buy U.S. oil under an annual contract, in a deal that will also help boost trade between New Delhi and Washington.