Stir oats with milk. Cover; let stand for 20 minutes.
Preheat oven to 350°F (180°C). Butter two 9-inch (23cm) round cake pans and line bottom with parchment paper. Stir flour with baking soda, cinnamon, allspice, ginger, nutmeg and salt. Set aside.
Blend oat mixture with butter, brown and granulated sugars, molasses, juice and vanilla using an electric mixer. Beat in eggs, one at a time. Stir in flour mixture. Stir in raisins. Divide batter between the two pans and bake for 30 minutes or until wooden pick inserted in centers comes out clean. Cool in the pans on a rack.
Frosting: Beat butter with corn syrup until smooth. Blend in sugar. Beat lemon juice until frosting is fluffy. Frost cake.