Preheat the oven to 425F and line 18 muffin tins with paper liners (or spray them with nonstick baking spray).
To make the streusel topping: In a small bowl, mix together the flour, brown sugar, and melted butter until the mixture resembles wet sand. Set aside until ready to use.
In a large bowl, whisk together the granulated sugar, melted butter, and vegetable oil.
Whisk in the milk, eggs, and vanilla.
Finally, whisk in the flour and baking powder. When the flour is almost fully incorporated, add the peaches and gently fold them into the batter (use a rubber spatula or large spoon, do not use a whisk).
Divide the batter evenly between the prepared muffin cavities, filling each one completely full.
Evenly divide the topping between the muffins and sprinkle on the top of each one.
Bake at 425F for 5 minutes. Then reduce the oven temperature to 350F and bake for an additional 15 minutes.
Remove the muffins from the oven and allow to sit a few minutes in the muffin tin. Then transfer to a wire cooling rack and allow them to cool completely before storing.