Make the streusel: in a medium bowl, whisk the flour, oats, brown sugar, sugar, salt, baking powder, and cinnamon to combine. Add the butter and mix with your hands until the mixture forms moist clumps. Set aside.

Preheat the oven to 325°F/165°C with the oven rack in the center. Lightly grease a 9x5-inch/23x13-centimeter loaf pan with nonstick spray. Line the pan with parchment paper, allowing 1 to 2 inches/2 ½ to 5 centimeters of excess on two longer sides of the pan (this will help you unmold it later).

In a medium bowl, whisk the oil, butter, sugar, and brown sugar until well combined. Add the eggs and whisk until well incorporated. Whisk in pumpkin, sour cream, and vanilla.

Add the flour, baking soda, salt, cinnamon, cloves, and nutmeg, and mix until evenly combined. Scrape the batter into the prepared loaf pan.

Use the tip of a paring knife to draw a line down the center of the loaf (this will help it rise more evenly). Sprinkle the surface of the batter with turbinado sugar, if using. Top the loaf with the prepared streusel in an even layer.

Transfer the loaf to the oven and bake until a skewer inserted into the center comes out clean—or with just a few moist crumbs, 1 hour 15 minutes to 1 hour 30 minutes. If the loaf has browned sufficiently, tent it with foil for the remainder of bake time.

Cool the loaf in the pan for 15 minutes, then use the parchment to unmold the loaf to cool completely on a wire rack before slicing and serving.