Crude oil dropped back under $91 per barrel Friday as the dollar index rose and equities turned sharply lower on Wall Street. TransCanada Corp. said it shut down the Keystone pipeline with expectations that it would take three days to check out problems revealed during routine tests, MarketWatch reported. The supply news provided some support early, but that faded as the session moved into the afternoon. The dollar index dropped 0.35 percent to 79.63, putting added pressure on commodity prices. In New York, November delivery of West Texas Intermediate crude oil gave up $2.24 or 2.4 percent to $90.65 per barrel on the New York Mercantile Exchange. Natural gas prices lost 0.35 cents to $3.579 per million British thermal units. Home heating oil lost 1.99 cents to $3.1397 per gallon. Reformulated blendstock gasoline shed 7.5 cents to $2.6788 per gallon. At the pump, the average price for a gallon of regular unleaded gasoline was $3.715 per gallon, down from Thursday\'s $3.737, AAA reported.