Preheat oven to 350 degrees F. Spray 2 9-inch loaf pans with nonstick cooking spray. Line with parchment paper if you have it, for easy removal.
In a large bowl with a hand mixer, or of a stand mixer with the paddle attachment, combine oil, sugar, eggs and vanilla at medium speed; about 30 seconds. Stir in zucchini. (If you don't have a mixer, use a large mixing bowl and a wooden spoon).
In a separate medium sized bowl, sift together flour, baking soda, baking powder, salt and cinnamon. Stir into the wet mixture, just until combined. Stir in walnuts in desired.
Pour into prepared loaf pans and smooth the tops evenly.
Bake for 1 hour. Test for doneness at 50 minutes as oven temperatures can vary. Let cool 15 minutes in the pans, then cool completely on wire racks.
Notes
Nutritional information is approximate and does not include optional ingredients. It is calculated per loaf because the serving size will depend on how thick you slice it.
Each loaf should yield 10 slices, but that can vary depending on the thickness of each slice.
Walnuts are optional. Pecans or chocolate chips could also be used.