One of two tank trailers being hauled by a big-rig truck overturned just before noon Thursday on U.S. Highway 50A, south of the Reno-Fernley Raceway between Fernley and Silver Springs, spilling an undetermined amount of oil used in road-asphalting jobs onto the dirt along the side of the road.For three hours, the southbound lane was closed and traffic alternated in the northbound lane. The highway reopened in both directions at 3:45 p.m., according to the Nevada Highway Patrol.The tractor-trailer was rounding a curve when the right rear tires of the rear trailer went off the shoulder. The driver swerved to …