Oil dropped to the lowest level in more than 5-1/2 years after Goldman Sachs Group and Societe Generale reduced their price forecasts. West Texas intermediate decreased 4.7% to $46.07 a barrel, and Brent 5.3% to $47.43. Crude has to “stay lower for longer” if investment in shale is to be curtailed to re-balance the global market, according to Goldman analysts.