Preheat oven to 350 degrees. Pour oil into a 13x9x2-inch pan. Tilt pan to cover bottom. Put cake mix, eggs and water into pan. Stir with a fork or spoon until blended, about 2 minutes. Scrape sides of pan and spread batter evenly. Spoon pie filling onto batter. Use a fork to fold into batter, just enough to create a marbled effect. Bake at 350 degrees for 30-35 minutes. Insert wooden pick into center. When it comes out clean, cake is done. When cool, sprinkle confectioners' sugar on top, if desired. Cover loosely.