Zhang Feng, an expert on lubricating oil at a research institute in Beijing, dined twice at a hotpot restaurant in Chengdu, Sichuan province, during his nearly five-day stay in the city.

"The hotpot was one of the elements that took me to Chengdu as a tourist with my wife and daughter in the middle of the month," he said. But he added that he had misgivings. He worried that the oil in the hotpot might not be as good as it appeared.

According to Li Zhiqiang, a restaurateur in Chengdu, "Recycled oil is used as a soup base in many hotpot restaurants to enhance the aroma."