* February synthetic $1.50/bbl under WTI * Players sell off remaining Feb barrels, sources say * February WCS quoted at $36.30/bbl under WTI CALGARY, Alberta, Jan 18 (Reuters) - Canadian synthetic crude prices fell to a three-week low on Friday as players sold off remaining barrels at the end of the monthly trading window, market sources said. Light synthetic for February delivery was last quoted at $1.50 a barrel under benchmark West Texas Intermediate, compared with premium of 15 cents on Thursday, according to Shorcan Energy Brokers. That was its deepest discount since December 27. "(There is) just some length on the market still," a trader said. The oil, upgraded from Alberta tar sands bitumen, had held strong this month while the discount on heavy crude deepened to multi-year lows due to a combination of tight pipeline capacity and delays in a U.S. refinery conversion project. Nominations for space on Enbridge Inc's network for February are due later on Friday. This month, the company imposed mid-month apportionment on three of its lines in Canada and the United States, further tightening already-limited capacity to export Canadian oil. Western Canada Select heavy blend for February was quoted at $36.30 a barrel under WTI, compared with a Thursday settlement of $37 a barrel under.