In a large bowl, sift together flour, baking powder, salt and granulated sugar. Stir in brown sugar. Mix well. With a pastry blender, cut in shortening until evenly distributed. Stir in oats and mix well. Put in large airtight container. Label as Oatmeal Mix and stor in a cool, dry place. Use within 10 to 12 weeks. Makes about 9 cups Oatmeal Mix.