If you want to understand why oil is back below $50 a barrel and likely headed much lower once summer ends, start with PetroChina PTR -1.50 % ’s stock prices. Mainland-listed shares in China’s national oil champion are up 27% so far this year, while their Hong Kong-listed equivalents are down 9%. The former have, of course, been juiced by Beijing’s desperate measures to prop up the mainland stock market. The latter are more reflective of what is really happening with oil supply and demand. China is showing signs of strain. While official gross-domestic-product data continue to helpfully meet Beijing’s targets, other numbers—and the stock-market panic— point downward. The latest, preliminary reading of the Caixin China Manufacturing Purchasing Managers’ Index hit a 15-month low . The State Council promptly announced measures to boost trade. Broad-based drops in the prices of industrial commodities from iron ore to copper serve as […]