TORONTO, Jan 18 Canada's main stock index fell on Monday as energy shares exerted further pressure on the overall index after crude oil prices hit 2003 lows, while Suncor Energy Inc slipped after upping its offer for Canadian Oil Sands Ltd. The pair of oil companies were among the most influential movers on the index, with Canadian Oil Sands surging 12 percent to C$8.38 and Suncor off 3.8 percent at C$30.04. Another major producer, Canadian Natural Resources Ltd , declined 2.3 percent to C$23.88.