In a large mixing bowl, dissolve yeast in warm water. Add the Sourdough Starter, oil, sugar, salt and 3 cups flour. Beat until smooth. Stir in enough additional flour to form a soft dough., Turn onto a floured surface; knead gently 20-30 times (dough will be slightly sticky). Place in a greased bowl, turning once to grease top. Cover and let rise in a warm place until doubled, 1 to 1-1/2 hours., Preheat oven to 400°. Punch dough down. Turn onto a lightly floured surface; divide in half. Roll each into a 12x8-in. rectangle. Roll up, jelly-roll style, starting with a long side; pinch ends to seal. Place seam side down on 2 greased baking sheets; tuck ends under. Cover and let rise until doubled, about 30 minutes. , With a sharp knife, make 4 shallow diagonal slashes across top of each loaf. In a small saucepan, combine water and cornstarch. Cook and stir over medium heat until thickened. Brush some over loaves. , Bake for 15 minutes. Brush loaves with remaining cornstarch wash. Bake until lightly browned, 5-10 minutes. Remove from pans to wire racks to cool.