Makes 4 round loaves
For sponge: Whisk all ingredients in large bowl until smooth. Cover bowl with plastic. Let rise in warm draft-free area until doubled, about 1 hour.
For dough: Using wooden spoon, stir down sponge. Add oil, salt and 3 cups bread flour, stirring until smooth. Mix in enough of remaining flour 1/2 cup at a time to form soft dough. Knead on floured surface until dough is no longer sticky, adding more flour if necessary, about 10 minutes.
Grease large bowl. Add dough, turning to coat entire surface. Cover bowl with plastic. Let dough rise in warm draft-free area until doubled in volume, about 1 1/2 hours.
Grease baking sheets. Gently knead dough on lightly floured surface until deflated. Cut into 4 even pieces. Knead each piece into round. Place on prepared sheets. Cover with towel. Let rise in warm draft-free area until doubled in volume, about 1 hour.
Preheat oven to 350 F. Bake until loaves sound hollow when tapped on bottom, about 1 hour. Immediately transfer to racks and cool completely.