Combine the water, starter, sugar, and oil in a large bowl. Add both flours and the salt and mix until no dry flour remains. Mix with your hands if it becomes too difficult to do with a spoon. Cover and rest for 15-30 minutes, then perform a round of stretch and folds to form the dough into a tight ball. This is a stiff dough so it won't do very much stretching at this stage - that's okay! Once you've gotten it into a ball, cover and let rise at room temperature until doubled in size - about 12-14 hours.Grease an 8 by 4-inch loaf pan. Turn the dough out onto a lightly floured surface and shape into a log (see post if you want details on this). Place seam side-down in the prepared pan and cover. Let rise until the dome comes up 1-inch above the rim of the pan, about 1-3 hours. Bake in a 375 degree oven for 45-50 minutes, until baked through and browned.Cool for 10 minutes in the pan, then invert to release and cool completely on a wire rack.
Notes
The oil - Any neutral oil will work here. My go-to's for this are avocado oil and olive oil. Vegetable oil will also work well.