The Amex Oil Index fell 13.2 points to 1,418.84 as petroleum futures fluctuated. Oil company shares have generally fallen this week as investors took profits from recent share run-ups driven by crude futures rising above $80 a barrel.

Investors struggled with the implications of an Energy Department report on Wednesday that showed unexpected increases in crude-oil inventories and declines in supplies of gasoline and distillates.

Light, sweet crude for November delivery rose 16 cents to $80.21 a barrel on the New York Mercantile Exchange, but alternated between gains and losses. November gasoline futures rose 2.27 cents to $2.0055 a gallon, and heating oil rose 1.47 cents to $2.177 a gallon as November natural gas futures fell 13.5 cents to $7.292 per 1,000 cubic feet.

The surprise build in crude supplies would seem to undermine high oil prices, however, much of that increase took place on the West Coast, where most gas and oil infrastructure is isolated from the rest of the country. At the same time, the decline in gasoline inventories would seem to support higher prices, but demand is falling.

Bear Stearns analyst Nicole Decker called it a "moderately bearish report for crude and moderately bullish report for refined product" in a note on Wednesday.

Royal Dutch Shell PLC fell $1.45 to $79.71, as ConocoPhillips fell $1.36 to $84.11 and BP PLC fell 87 cents to $67.87. Exxon Mobil Corp. shares fell 96 cents to $91.34 and Chevron Corp. dropped 80 cents to $91.76.