U.S. crude oil fell to its lowest in almost six-and-a-half years on Friday as huge stockpiles and refinery shutdowns heightened concerns about global oversupply and slowing economies in Asia. Oil had already tumbled more than 3 percent on Thursday, driven by a report that stocks at Cushing, Oklahoma, the delivery point for U.S. crude futures, rose more than 1.3 million barrels in the week to Aug. 11. U.S. crude was down 24 cents at $41.99 a barrel by 0945 GMT.