Heat oven to 400ºF. Spray bottoms of 18 medium muffin cups with cooking spray or line with paper baking cups. For topping, combine all topping ingredients in medium bowl; mix well. Set aside. For muffins, in large bowl, mix together milk and oats. Let stand 10 minutes. Stir in oil and eggs until well blended. Add banana quick bread mix and dried plums. Stir by hand 50 to 75 strokes, as directed on package, or until blended. Divide batter evenly among muffin cups, filling each about 3/4 full. Sprinkle with topping, dividing evenly. Bake 15 to 20 minutes or until wooden pick inserted in center comes out with a few moist crumbs clinging to it and topping is golden brown. Remove muffins from pan to wire rack. Serve warm or at room temperature.
To freeze, wrap cooled muffins in foil, or place in freezer bag. Seal, label and freeze up to 6 months.