Preheat oven to 350°F (175°C) and line a muffin tin with paper liners.
In a large bowl, whisk together pumpkin puree, eggs, brown sugar, oil, milk, and vanilla.
In a separate bowl, combine flour, oats, baking soda, salt, cinnamon, nutmeg, and cloves.
Add the dry ingredients to the wet ingredients and mix until just combined.
Fold in walnuts if using.
Divide batter evenly among muffin cups, filling each about 3/4 full.
Bake for 20–25 minutes or until a toothpick inserted into the center comes out clean.
Cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.