OPEC’s biggest producers probably don’t need to worry about their sales being challenged by U.S. crude exports to Asia, yet. U.S. West Texas Intermediate oil must drop $4 to $6 below Dubai crude, the benchmark for Middle East supply, according to South Korea’s Ministry of Trade, Industry and Energy. It also said American cargoes must become cheaper relative to Brent, the marker for shipments from regions including Africa, for them to be viable. South Korea is the fourth-biggest consumer in Asia, the world’s largest oil market.