Combine Sourdough Starter, water, oil, sugar, salt, and 3 cups flour in a large mixing bowl; beat with an electric mixer until smooth. Stir in enough remaining flour to make a soft dough.
Turn dough out onto a floured surface, and knead 10 minutes or until smooth and elastic. Place dough in a greased bowl, turning to grease top. Cover and let rise 6 hours or until doubled in bulk.
Punch dough down; turn out onto a lightly floured surface. Divide in half, shaping each half into a loaf. Cover and let rest 5 minutes. Place loaves in 2 greased 9- x 5- x 3-inch loaf pans. Cover and repeat rising procedure 3 hours or until doubled in bulk. Place in a cold oven. Bake at 350° for 40 minutes or until loaves sound hollow when tapped. Remove bread from pans, and cool on wire racks. Brush tops with melted butter.