Pumpkin Muffins

INGREDIENTS

• 3/4 cup vegetable oil, plus more for pan
• 1 1/2 cups whole-wheat flour, spooned and leveled
• 1 1/2 cups all-purpose flour, spooned and leveled
• 2 teaspoons baking powder
• 1 tablespoon pumpkin pie spice
• 1/2 teaspoon baking soda
• 2 cups pumpkin puree
• 1 cup plain low-fat yogurt
• 3 large eggs
• 1 cup turbinado sugar, plus 2 tablespoons more for sprinkling
• 1 1/2 cups coarsely chopped walnuts