New York's main contract, light sweet crude for September delivery, sank $1.13 to $90.70 a barrel while Brent North Sea crude for delivery in September eased 86 cents to $105.97.
Spain's ability to handle its finances is spooking investors this time amid worries that the country, the fourth largest economy in the eurozone bloc, would need a bailout.
The country was forced to pay borrowing costs above 7.0 percent in a 10-year bond sale, a level considered unsustainable and that pushed Greece, Ireland and Portugal into asking for bailouts.
"The eurozone crisis has become the focus again," Singapore's DBS bank said in a commentary.
"The market's main fear is that Spain may require a full bailout some time in the future," it said.
Crude futures had soared in recent sessions, hitting two-month highs Thursday, as traders worried that rising tensions in the oil-rich Middle East will hamper supplies.