New York's main contract, light sweet crude for delivery in January shed 25 cents to $87.13 a barrel in the afternoon and Brent North Sea crude for January delivery sank 30 cents to $110.25.
"Oil dipped in thin trade, as a ceasefire in the Gaza Strip eased supply concerns," Phillip Futures said in a report.
"Gloomy manufacturing data for Europe" also pulled down prices, he said.
Gaza was gripped by an uneasy peace after the Egyptian-brokered ceasefire came into effect late Wednesday and halted eight days of conflict between Israel and Hamas.
Israel Prime Minister Benjamin Netanyahu said Thursday his government was "giving the ceasefire a chance" but was prepared for the eventuality it could collapse.
In Europe, a closely-watched survey by research firm Markit showed the troubled region's manufacturing activity in November little changed from lows experienced in October.
Markit's eurozone Purchasing Managers Index (PMI) for the month of November stood at 45.8 points, "up fractionally from 45.7 in October... October's reading had been the lowest since June 2009," the survey's report said.
"For the fourth quarter of 2012 so far, PMI data suggest the strongest contraction of output since the second quarter of 2009."
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