To the bowl of a stand mixer with a dough hook add the bubbly leaven, water, sugar, salt, and vegetable oil.
Add 5.5 cups flour and the vital wheat gluten and mix. Continue adding flour until the dough pulls away from the side of the bowl, the dough is tacky (but not overly sticky) and you can pinch of a piece of dough, roll it in your fingers and just have a little bit of residual dough on your fingers. Check out these tips to know if your dough is ready.
Knead the dough for about 8-10 minutes (set a timer and let the mixer go) and add a Tablespoon of flour at a time as needed.
Transfer the dough to an oiled bowl, cover with plastic wrap and let rise for 8-12 hours (overnight).
The next morning the dough will have risen. The amount of rise depends a lot on the temperature of your kitchen. Don't worry, if your starter is very active, it will be okay.
Transfer the dough to a countertop. Cut the dough in two sections for two large loaves or in three for three smaller loaves.
Pat the dough into a rectangle and roll up cinnamon-roll style, pinching the seams closed as you roll.
Repeat with the remaining sourdough loaves.
Place the loaves on a parchment-lined baking sheet or use a baguette bread pan. Cover with a kitchen towel or plastic wrap. Let the loaves rise for 3-6 hours until puffy and almost doubled in size (the time will depend on the warmth of your kitchen).