Crude-oil futures fell Friday for a sixth straight session to their lowest level in four years, ending the week with the biggest loss since the Persian Gulf War in 1991, as worse-than-expected U.S. jobs data darkened the outlook for energy demand. Crude oil for January delivery ended down $2.85, or 6.5%, at $40.81 a barrel on the New York Mercantile Exchange, the lowest closing level since December 2004. Oil lost 25% during the week, the largest percentage drop since the week ended Jan. 18, 1991.