Dissolve yeast in the warm water; add the molasses. Stir in the buckwheat flour and 1 cup whole wheat flour, beating vigorously for about 50-70 strokes. Cover with a cloth and place in a warm spot for about 30 minutes. This will make a sponge.
Fold in the salt and oil, not cutting into the mixture, but turning and folding the whole mass over. Work in the remaining 3 cups of whole wheat flour and the 1-2 cups unbleached white flour as need to prevent it from sticking.
Turn out onto a floured work surface; knead until smooth (5-10 minutes). Place dough in a greased bowl; turning once to coat; cover and let rise 60-90 minutes. Deflate dough; shape into 2 loaves.
Place in greased loaf pans. Cover and let rise again until doubled.
Preheat oven to 375 degrees F and bake on middle rack of oven for 35-40 minutes. (Rub with butter for a soft crust.) Remove loaves from pans and cool on rack.
