Sift flour onto baking board. Make a depression in center of flour and add oil, salt and enough water (approximately 1 cup) to knead a smooth, elastic dough. Cover dough with a warm bowl and let stand for about 20 minutes. Meanwhile, cut apples in thin slices. Add sugar, raisins and cinnamon and mix well. Roll dough as thin as possible on a board or other flat surface covered with flour. Carefully lift rolled dough from board and place flat on a smooth, clean dishcloth covered with flour. Brush dough with sour cream and spread with filling. Roll up with help of the dish towel by lifting one end so that strudel rolls itself up. Press dough together at both ends. Brush strudel with egg yolk and bake at 325° for approximately 40 to 50 minutes.