Preheat the oven to 180ºC . Spray or wipe a 12-cup Bundt pan with vegetable oil. Lightly dust the pan with flour. In a large mixing bowl, cream the sugar, half cup oil, and butter until fluffy. Add the dry ingredients and mix until blended. Add the egg, egg whites, vanilla, and buttermilk, and blend well. Stir in the plums and walnuts; pour the batter in the pan and bake for 50 minutes, or until a cake tester or toothpick comes out clean. While the cake is baking, in a saucepan heat the remaining half cup canola oil and brown sugar until hot. Add the skim milk, stir, and set aside. Cool the cake in the pan, then invert it onto a plate, and spoon the glaze over the top. Cal. 266, Carb. 36g, Protein 3g, Chol. 13mg, Fat 12g/41%