This recipe is adapted from A Blessing of Bread. It even calls for icing on the challah but I thought that was gilding the lily.

7 teaspoons yeast
3-1/3 cups warm water
1-2/3 cups sugar
9 large eggs
5 large egg yolks
2 tablespoons salt
1 cup oil
1 5-lb. bag bread flour

Proof yeast in warm water. Pour in sugar. After yeast has bubbled, add eggs and yolks, salt and oil. Whisk well. Stir in flour, starting with 4 cups at a time and gradually decreasing the amount as the challah starts to lose its stickiness. Continue to knead in flour until dough is smooth and elastic. Roll in about 1 teaspoon oil and cover with a damp cloth. Allow to rise for about 2 hours. Divide into 6, braid and place on greased baking sheets or in loaf pans. Cover and let rise for another hour. Bake at 350 degrees for 30-40 minutes.