US refineries have seen their stock prices drop by as much as 10% this week after the Commerce Department gave Pioneer Natural Resources (NYSE:PXD) and Enterprise Products Partners LP (EPD) permission to export condensate, a rare exemption from the 40-year-old oil export ban. But this market swing may overestimate the important that the changing policy actually has on the US oil sector. “In general, lifting the ban would be slightly positive for US condensate and oil producers, but slightly negative for US refiners who are investing in condensate splitters and benefit from local crude discounts,” write Oppenheimer analysts Fadel Gheit…