Buy low, sell high. That's been a formula for success for petroleum refiner Delek US Holdings (DK) for almost two years. Unlike refiners on the Gulf Coast, its inland refinery locations give it the ability to buy more discounted Midland, Texas-sourced crude oil, pegged to West Texas Intermediate (WTI) prices. "Midland crude is one of the cheapest crudes in North America right now and both of our refineries are enjoying access to it," said Assi