Crude oil costs European companies the same as American companies, with the price set in dollars. The biggest reason gasoline is so much more expensive in Europe is very high taxes. Sure, they may have some higher costs associated with refining oil there, but no where nearly enough to account for the difference between US and European prices. From this MSNBC article (date isn't certain, but it appears to be from last year):

Taxes on fuel are of course the big difference between Europe and the United States. U.S. gas taxes average about 40 cents a gallon; European taxes are about $2 a gallon. In Britain, the current excise tax per gallon is $3.40, says Ian Parry of Resources for the Future, an environmental think-tank based in Washington. That stunningly high tax is defended by Prime Minister Tony Blair’s government as necessary for reducing pollution, easing traffic congestion and encouraging consumers to purchase more fuel efficient cars.

Notice that I specifically mentioned high gasoline prices rather than petroleum prices. The European company Thielert Aircraft Engines has certified a diesel engine for light aircraft, primarily as a retrofit engine for the Piper Warrior and Cessna 172 but also original equipment on the Diamond Twinstar. Aviation gasoline is even more expensive in Europe than it is in the US (where avgas has sold for over $2 a gallon for years). A diesel engine can use jet fuel (kerosene) or diesel fuel. While the diesel engine does have higher fuel efficiency (about 25% better), converting a gasoline lightplane to diesel can lower operating costs by 70%, with lower taxes on jet and diesel fuel being a major factor.