Prepare rice mix according to package directions, except do not add oil or butter. Cool.
In a large bowl, stir together rice mixture, water chestnuts, celery, green onions, and grapes. In a screw-top jar, combine olive oil, lemon juice, and pepper. Cover; shake well. Pour lemon juice mixture over rice mixture; toss to coat.
Cover and chill in the refrigerator for at least 4 hours or up to 24 hours. To serve, gently stir in mandarin oranges and pecans. Makes 8 to 10 side-dish servings.