Heat oil in a large saucepan over medium-high heat and add leeks. Saute for 5 minutes or until tender. Add potato and broth; bring to a boil. Reduce heat and simmer 25 minutes or until potato is tender. Place 3 cups potato mixture in a blender. Remove center piece of lid to allow steam to escape. Place a clean towel over the lid and blend until smooth. Return pureed mixture to pot, bring to simmer. Stir in arugula, salt & pepper, cook 2 minutes or until arugula wilts.