Using a stand mixer with a dough hook or in a large bowl, add starter, flour, salt, sugar, oil, and warm water.
If using a stand mixer, knead for 5-10 minutes until dough is smooth and elastic. If mixing by hand, knead on a floured surface for 10-12 minutes or until dough is smooth and holds its shape.
Place in a greased bowl and cover with a damp dish towel. Place in a draft-free area (a microwave or unheated oven works well). Let dough rise at least 12 hours or overnight.
Punch down and turn dough onto a floured surface. Knead for several minutes to remove any air bubbles.
Spray 2 loaf pans with cooking spray. Divide dough in half, shape the loaves and place into the pans. Cover and allow the dough to rise until it has doubled in size, about 6-8 hours.
When ready to bake, preheat oven to 350° F (177° C).
Bake for 30 to 35 minutes until the tops are golden and the bread has pulled away from the sides of the pan. If desired, brush the tops with butter 5-10 minutes before removing from oven.