Oil headed for its biggest weekly gain in over two years on hopes that OPEC will manage to shrink a glut and trade tensions between the U.S. and China will ease.Futures in New York have advanced 9.5 percent this week, as Saudi Arabia pledged that a producer coalition it’s leading will keep the market in balance. Still, prices are about 30 percent lower than their highs in October even after a rebound since Christmas Eve thrust crude back into a bull market. That signals investors need reassurance that the group will curb supply sufficiently and demand will hold up.