SINGAPORE (Reuters) - Oil prices slumped to 2018 lows on Friday in thin but volatile trading, pulled down by concerns of an emerging global supply overhang amid a bleak economic outlook.Even an expectation that the Organization of the Petroleum Exporting Countries (OPEC) producer group will start withholding supply in 2019 to rein in any glut provided little support, traders said.International benchmark Brent crude oil futures hit their lowest since December 2017 at $61.52 per barrel, before..