Cream together butter or margarine, vegetable oil and sugar. Add eggs, sour milk, and vanilla. Blend well. In a separate bowl, sift dry ingredients together. Blend dry ingredients into the butter mixture. Add grated zucchini, and mix well. Spread in a greased floured 9 x 13 inch pan. Sprinkle walnuts and chocolate chips on top. Bake in a preheated 325 degrees F (165 degrees C) oven for 45 minutes.