Preheat the oven to 350°F (175°C). Lightly coat a 9 × 5-inch (23 × 13 cm) loaf pan with baking spray to help prevent sticking. Set the prepared pan aside while making the batter.

Place the melted butter, vegetable oil, packed brown sugar, room-temperature eggs, water, and vanilla extract in a large mixing bowl. Whisk until the ingredients are thoroughly incorporated and the mixture appears smooth.

In a separate medium bowl, whisk together the all-purpose flour, baking soda, salt, cinnamon, ginger, nutmeg, and cloves. Continue whisking until the spices and leavening are evenly dispersed throughout the flour.

Add the dry mixture to the wet mixture gradually, working in approximately three additions. Whisk gently after each portion and stop as soon as the ingredients are incorporated to avoid overmixing. The completed batter should have a smooth, thick consistency.

Transfer the batter to the prepared loaf pan and spread it evenly. Peel the ripe pears and cut them into slices. Position the pear slices upright across the surface of the batter in an alternating arrangement. Scatter the rolled oats evenly over the top.

Place the loaf pan in the preheated oven and bake for 40–45 minutes. The cake is ready when the surface is golden brown and a toothpick inserted into the center comes out clean. Check the loaf toward the end of the baking period to prevent overbaking.

Remove the spiced pear cake from the oven and leave it in the pan for at least 10 minutes. Carefully transfer the loaf to a wire cooling rack and allow it to cool completely. Once cooled, lightly dust the surface with powdered sugar if desired, then slice and serve.