Combine raisins and rum in a small microwave-safe bowl; microwave at HIGH for 30 seconds. Cool to room temperature.
Weigh or lightly spoon flour into dry measuring cups; level with a knife. Combine flour and next 3 ingredients (through salt) in a bowl; stir with a whisk. Place butter in a large bowl; beat with a mixer at medium speed until smooth. Add oil, 1 cup granulated sugar, rinds, and vanilla; beat at medium speed 3 minutes or until light and fluffy. Add eggs, 1 at a time, beating well after each addition. Add flour mixture and milk alternately to butter mixture, beginning and ending with flour mixture. Drain raisins through a sieve over a bowl; reserve liquid. Stir raisins into batter. Pour batter into a 10-cup Bundt pan coated with baking spray. Bake at 350° for 40 minutes or until a wooden pick inserted in center comes out clean. Cool in pan 10 minutes on a wire rack. Remove from pan; place on serving plate.
Combine remaining 1/3 cup granulated sugar, corn syrup, and 2 tablespoons water in a small saucepan; bring to a boil. Cook 1 minute. Remove pan from heat; stir in reserved rum. Brush syrup over warm cake. Cool completely. Sprinkle with powdered sugar.
I bought a mini bundt cake pan to make this recipe, and I had way too much batter. The mini bundt cakes came out looking like weeble-wobbles. Upside down they looked like huge muffin tops. But, that's not to say the cakes weren't delicious. They were. I used a regular golden rum, which didn't add enough flavor, so next time I'll make it with a good dark rum. Instead, a delicate vanilla flavor dominated, which was irresistable. When I make this again, I'll either purchase another mini bundt cake pan for the overflow or put the excess batter in a mini-loaf pan(s).