1. Preheat oven to 350°. Oil and flour a 12-cup Bundt pan.
2. Beat eggs and 1 1/2 cups sugar in a large bowl with a mixer on medium speed until thoroughly combined, about 30 seconds.
3. Add 1 cup Roussanne, the tangerine olive oil, vanilla, and tangerine zest. Beat on low until blended, about 30 seconds.
4. Whisk together 2 1/2 cups flour, salt, and baking powder in another bowl. Add flour mixture to egg mixture and beat on low speed to incorporate. Increase speed to medium and beat 30 seconds, then scrape down sides of bowl. Increase speed to medium-high and beat 30 seconds more to blend well. Pour batter into prepared pan.
5. Bake cake until a wooden skewer inserted in the center comes out clean, about 30 minutes. Let cake cool on rack, 5 minutes. Carefully run a thin knife between cake and pan to loosen, then invert cake onto rack.
6. Peel whole tangerine with a vegetable peeler. In a small saucepan, bring remaining 1/2 cup sugar, 1/2 cup Roussanne, and the tangerine peel to a simmer over medium heat and simmer until sugar has dissolved completely, about 5 minutes.
7. Poke deep holes in cake (about 50 holes spaced 1/2 in. apart) with a thin skewer.
8. Remove peel from hot syrup and spoon syrup over still-warm cake. Let cool completely and serve with whipped cream if you like.
Note: Nutritional analysis is per slice.