The weakness came as oil traders forecast the fourth straight increase in crude oil inventories in the United States, the world's largest energy consumer, as imports flooded to shore and refiners slowed rates for maintenance work.
US oil supplies are already about 9 percent higher than last year. The US Department of Energy will release its fresh weekly inventory report Wednesday.
"Predictions of an increase in crude stocks in tomorrow's data has sparked selling here, but even at this level, prices are still high," said a NYMEX floor trader.
The Organisation of the Petroleum Exporting Countries decided at a meeting Sunday to keep official production limits steady despite high prices, and ministers signalled a cut may be needed soon to defend against a decline in demand.
Opec President Sheikh Ahmad al-Fahd al-Sabah of Kuwait said if inventories build too quickly and prices fall he could call a ministerial teleconference to take action before the groups' next meeting in Iran in mid-March.
The cartel also formally abandoned its $22-$28 price target for its basket of crudes and appears set to defend $40 by leaving the door open to cutting production later to contain a springtime stockbuild.
"Opec has come clean about its real intention: to test the new level of oil prices," said PFC Energy in a report.
The price of Opec's basket of crudes fell to $40.95 a barrel on Monday, down from Friday's $41.88.
US crude oil inventories are running above year-ago levels and gasoline stockpiles are up 3 percent, although fires at two US refineries have raised concerns over motor fuel supplies months before the peak summer season.
Analysts expected a government report due Wednesday to show US crude inventories rose 1.5 million barrels in the week to January 28. If correct, US crude tanks will have risen for four straight weeks.
"We will continue to see a build-up of US commercial crude stocks as refiners go into turnaround maintenance and imports remain high," said brokers Refco in a report.
Analysts also forecast US distillate stocks, which include heating oil and diesel, to fall by 2.4 million barrels after a cold snap hit the Northeast region, the world's largest consumer of winter heating fuels.
Temperatures are forecast to return to seasonal norms this week and remain warmer than usual the rest of winter, easing pressure on low fuel inventories.