Heat oven to 350°.
Grease and flour baking pan, 13x9x2 inches.
Measure all ingredients except topping into large mixer bowl.
Mix on low speed, scraping bowl constantly, 1/2 minute.
Beat on high speed, scraping bowl occasionally, 3 minutes.
Pour into pan.
Bake until wooden pick inserted in center comes out clean, 35 to 40 minutes; cool slightly.
Spread topping on cake.
Set oven control at broil and/or 550°.
Place top of cake 3 inches from heat.
Broil until topping is golden brown and bubbly, 2 to 3 minutes.
(Can be served immediately.) Cool cake 1 hour.
Freeze uncovered 2 hours.
Wrap, label and return to freezer.
30 minutes before serving, remove Oatmeal Spice Cake from freezer and loosen wrapper.
Thaw at room temperature.
(If plastic wrap has been used, place wooden picks at intervals to prevent wrapper from sticking to topping.)