New York's main contract, West Texas Intermediate crude for delivery in June, shed 95 cents from Thursday to close at $96.13 a barrel.
Brent North Sea crude for June settled at $112.26 a barrel, down 47 cents in London trade.
"The week we had for crude has been just distinctly bearish," said Matt Smith at Summit Energy.
On Friday, Greece's socialist leader admitted he had failed in a last-ditch bid to form a government, taking the nation a step closer to repeat elections as it faces increased European Union pressure over its finances.
Socialist party boss Evangelos Venizelos was the third party leader who tried and failed to cobble together a government after inconclusive elections Sunday that saw a backlash against painful austerity measures.
The latest twist in the tortuous political drama came as EU paymaster Germany threatened to cut off the country's loan lifeline and hinted that the crisis-ridden eurozone could get along without Greece.
Fresh signs of slowing of growth in China, the world's biggest energy consumer, also damped market sentiment.
China said Friday that industrial output growth hit a three-year low in April, adding to pressure on Beijing to ease monetary policy.
Official data showed industrial output rose 9.3 percent last month, the slowest pace since May 2009 and well below expectations.
"The Chinese industrial production numbers really knocked the market," said Smith.
"China's economy is even weaker than thought, with industrial production growth back in single digits for the first time since the global financial crisis," said Ren Xianfang, an economist at IHS Global Insight.
The International Energy Agency's edged up its outlook for oil demand growth this year but warned that of geopolitical risks over Iran's disputed nuclear program.
In its monthly report, the IEA said that despite recent oil market falls, "the path of market fundamentals for the rest of the year remains highly uncertain and geopolitical risks will likely continue to keep prices high."
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