Oil prices advanced slightly on Monday, as China's gross domestic product (GDP) in the second quarter came in as expected and U.S. economic data turned out mixed. China's GDP grew at an annual rate of 7.5 percent in the second quarter, slowing down from 7.7 percent in the first quarter, official data showed Monday. However, the figure was inconsistent with market consensus, thus easing market worries about oil demand in the world's second largest economy. The oil market also gained some momentum from news of a mechanical glitch in an Iraq/Turkey pipeline.