In refined products, NYMEX November ULSD settled down 1.2 cents at $2.4856/gal. NYMEX November RBOB closed 3.25 lower at $2.2002/gal.
Crude futures managed to avoid falling to multi-year lows seen last week, but were still unable to close in positive territory. "Oil prices had a hard time maintaining any kind of rally," Phil Flynn, analyst at Price Futures Group, said.
A downturn in the US equity markets, combined with news out of Iran that the government is not calling for an emergency OPEC meeting, helped push oil prices lower, he said.
Another issue emerging Monday was the US Department of Energy's upcoming review on the Strategic Petroleum Reserve, Flynn said.
One conclusion reached could be the following: "Why have an SPR when we have North Dakota?" he said.
An agency letter released publicly Monday said the DOE will be looking at a range of issues related to the SPR, including its size, in light of changes in the US and world oil markets (See story, 1818 GMT).
The SPR currently holds 691 million barrels of crude in underground salt caverns in Louisiana and Texas and is authorized to hold up to 1 billion barrels.
Rising US crude production has been a major driver behind the slide in oil prices since the summer.
Another factor has been Libya, which has seen a recovery in production and exports, despite political turmoil and fighting.
Libyan output hit 925,000 b/d at the end of September before falling due to disruption by striking workers at eastern oil fields.
On Monday, Libyan crude production reached 890,000 b/d, up from 765,000 b/d earlier this month (See story, 1353 GMT).
Also in the Middle East, fresh data showed Saudi Arabia's exports fell to their lowest monthly volume since March 2011 (See story, 1210 GMT).
Saudi Arabia exported 6.683 million b/d in August, a 326,000 b/d drop from July, according to data from the Riyadh-based Joint Oil Data Initiative.
Weaker exports would seem to represent a counterpoint to the prevailing narrative that the world's largest oil exporter is more interested in cutting prices than output.
Though, one factor behind the lower exports was domestic demand. The amount of crude processed by Saudi refineries hit a record-high 2.167 million b/d in August as new refineries in Jubail and Yanbu have come online or ramped up.
"Looked at it from another perspective, the drop in prices over the past four months is simply the market's way of requesting a production cut," Citi Futures analyst Tim Evans said in a client note.
How much further oil prices will slide has become the focal point ahead of OPEC's next conference scheduled for November 27.
Analyst Philip Verleger said prices will continue falling until enough non-Middle Eastern production shuts in or until other producers understand that Middle Eastern countries will not accept a reduction in market share.
"The down trend is not over. It has just started," he said in a client note Monday.
This cycle is underway after Middle Eastern producers Kuwait, Iraq, Iran, the United Arab Emirates and Saudi Arabia slashed their discounts to oil benchmarks in every major market to ensure refiners will operate profitably, he said.
With an incentive to operate full-tilt, more products will enter the market, pulling product prices lower which will then drag down crude prices as well, Verleger said.
--Geoffrey Craig, email@example.com --Edited by Katharine Fraser, firstname.lastname@example.org