Preheat the oven to 425°. Pour the oil into a 9-by-13-inch baking dish and heat in the oven. In a large bowl, whisk the cornmeal with the flour, sugar, baking powder, salt and baking soda. In another bowl, whisk the buttermilk with the eggs. Pour the buttermilk mixture into the dry ingredients, add the melted butter and stir with a wooden spoon until just blended; do not overmix.