Oil posted a fourth weekly gain amid lower summer trading volumes and tentative signs that global markets are tightening. Russia has shown indications of paring crude exports as it belatedly honours a pact with Saudi Arabia and the OPEC+ alliance to help balance world markets. China, the world's biggest crude importer, also has stepped up efforts to boost its flagging economic recovery. This week, oil continued to follow the whims of broader market sentiment due to dwindling liquidity. Cru ....