In large bowl of an electric mixer, beat butter, shortening, granulated sugar, and brown sugar on high speed until very light and fluffy (about 5 minutes). Add eggs, one at a time, beating well after each addition. Beat in vanilla and lemon juice. In another bowl, stir together flour, baking soda, salt, cinnamon (if used), and oats. Gradually add to butter mixture, blending thoroughly. Stir in chocolate chips and walnuts (if used).
Use a scant 1/4 cup of dough for each cookie. Drop dough onto lightly greased baking sheets, spacing cookies about 3 inches apart. For soft cookies, bake in a 325° oven for 17-19 minutes or until light golden brown; for crisp cookies, bake in a 350° oven for 16-18 minutes or until golden brown. Transfer to racks and let cool. Store airtight.