HEAT oven to 350ºF.
MICROWAVE butter and peanut butter in large microwaveable bowl on HIGH 45 sec. to 1 min. or until both are completely melted; stir until well blended. Add sugar; mix well. Stir in oats; spread onto bottom of 17x11-1/2x3/4-inch pan.
BAKE 20 min., stirring every 5 min. Cool.
ADD trail mix; mix lightly.
