U.S. benchmark crude lost 15 cents per barrel to finish at $104.55 in New York. Brent crude, the benchmark for international crudes, rose 28 cents to end at $107.45 in London.
Crude prices have fallen somewhat after they rose quickly in early July to reach nearly $110 per barrel in trading on July 19.
Traders are looking for clues Wednesday in an announcement by the Federal Reserve on whether and how long the Fed will scale back its stimulus plan. The Fed has been buying $85 billion of financial assets a month in an attempt to keep long-term borrowing rates low and help shore up the U.S. economic recovery.
Low interest rates make it easier and more attractive for investors to buy commodities such as oil, bolstering prices.
Traders may still be left wondering, though. "We look for the status quo to be maintained with few clues," wrote analyst Jim Ritterbusch in a report Monday.
The stimulus program is widely expected to be scaled down later this year as the economy improves.
Traders are also looking to see whether Energy Department will report another draw in oil supplies after dropping by a surprisingly large 30 million barrels over the past month.
Oil supplies remain high compared with the five-year average, even after the monthlong reduction, so most traders say it is more likely that oil prices will soon fall than that they will rise further.
Analysts at Commerzbank added that the Nymex contract had developed a "considerable potential for correction, which points to further price falls in the coming days."
In other trading on the New York Mercantile Exchange:
-- Wholesale gasoline for September fell 3 cents to finish at $2.98 a gallon.
-- Heating oil for September rose less than a cent to end at $3.02 a gallon.
-- Natural gas for September fell 9 cents $3.47 per 1,000 cubic feet.