Brent falls 10 pct in October, WTI down 11 pct; Dollar strong after U.S. GDP data.

Brent crude oil fell more than a dollar towards $85 a barrel on Friday as a firmer dollar and a well supplied oil market pushed the benchmark towards its steepest monthly decline since 2012.

The U.S. dollar rose to a three-week high on Thursday, after data showed the U.S. economy grew 3.5 percent in the third quarter, topping market estimates for a 3 percent rise.

A strong dollar makes commodities such as oil more expensive for buyers using other currencies, suppressing demand.

"The market is asking how low the price must go before output is trimmed," said Bjarne Schieldrop, chief commodities analyst at SEB in Oslo.

Brent for December was down 85 cents at $85.39 a barrel by 1135 GMT, after touching a low of $85.11. The oil benchmark has fallen 10 percent so far in October, its biggest monthly drop since May 2012.

U.S. crude was down 60 cents at $80.52 per barrel, having lost 11 percent this month, also its worst performance since May 2012.

Outside the United States, other major economies show signs of slowing economic growth, suggesting a slowing demand for oil.

The Bank of Japan surprised global financial markets on Friday by expanding its stimulus spending, boosting Japanese equities but raising concerns about the economic health of the oil importer.

China's manufacturing sector is stable but faces relatively big downward pressure on growth, the Ministry of Industry and Information Technology said on Friday.

In contrast to slowing demand, global supply has increased, and the Organization of the Petroleum Exporting Countries has given no indication that it will cut output targets at a meeting on Nov. 27.

Hans van Cleef, senior energy economist at ABN AMRO in Amsterdam, told the Reuters Global Oil Forum that there would be "tough discussions" at the OPEC meeting, but that the output quota was likely to remain unchanged at 30 million barrels a day.

OPEC Secretary General Abdullah al-Badri said on Wednesday the cartel's output was unlikely to change in 2015, and said he was not concerned about falling prices.

"OPEC will be happy to see prices languish in a lower range in order to regain market share against U.S. producers, whose costs are higher," said Tony Machacek, a broker at Jefferies in London.

By Sam Wilkin