Preheat the oven to 425°. Pour the vegetable oil onto an 11-by-17-inch rimmed baking sheet and heat in the oven. In a large bowl, whisk the cornmeal, 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 until blended.