New York-traded West Texas Intermediate, the U.S. crude benchmark, settled down 62 cents, or 1.0%, at $61.06 per barrel. Despite that drop, WTI rose 11% for December, its largest monthly gain since January. London-traded Brent, the global oil benchmark, settled down 67 cents, or 1%, at $66.65 per barrel. Notwithstanding Tuesday’s slide, the U.K. crude standard settled up 7% for December, its largest monthly advance since April. Oil’s 2019 rally was largely helped by production cuts carried out by OPEC. Since January, the Saudi-led OPEC, joined by its ally Russia under the OPEC+ alliance, has tried to observe a daily production cut of 1.2 million barrels.     Despite its plan for stiffer production cuts, OPEC+ could have a tougher time keeping oil prices up in 2020 as U.S. shale oil output could rebound next year, some long-time traders in oil said. Non-OPEC oil supply, led by the U.S. shale, is forecast to grow by 2.1 million barrels a day in 2020, according to the Paris-based International Energy Agency (IEA). Global demand for oil, meanwhile, is set to increase by 1.2 million barrels a day next year, the EIA said.