try to get something that contains both the omega 3 & 6 essential fatty acids. Udo's Perfected Oil blend or hempseed oil have very excellent ratios of both...hope that helps
Actually, heavy metals are quite easy to remove, and this will not be a problem with most reputable brands. For PCBs to be removed, however, requires molecular distillation. Not many brands do this(ZonePerfect is the only one that immediately comes to mind.)