U.S. refiners, which posted robust profits the last 18 months even as other parts of the oil business were racked by low crude prices, finally saw their roll come to a halt in the first quarter. Many refining businesses reported earnings for the period that were down roughly by half from a year earlier. That decline helped sour results for oil giants such as Exxon Mobil Corp. XOM -1.14 % , which has counted on refining to offset profit declines in energy production, and for Valero Energy Corp. VLO -3.66 % , the world’s largest stand-alone refiner by output, which on Tuesday reported its lowest first-quarter profit in four years. “The first quarter presented us with challenging markets, with gasoline and diesel margins under pressure,” said Valero Chief Executive Joe Gorder. Two trends have […]