New York's main contract, West Texas Intermediate crude for delivery in July, slipped 72 cents to close at $84.10 a barrel.
Brent North Sea crude for July lost 46 cents, settling at $99.47 a barrel in London trade.
"It's a really confusing market, there is no big position," said Rich Ilczyszyn, a trader at iiTrader.
He said traders were keeping an eye on Spain, with expectations Madrid will seek some form of rescue for its banking sector over the weekend.
"A lot of players are outside the market" because of the uncertainties in the eurozone, he said.
"Nobody can quantify risks, because we don't know what the monetary and fiscal situation is."
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