Combine sugar, water, syrup, and salt in a small Dutch oven. Cook over medium heat, stirring constantly, until sugar dissolves. Cover and continue to cook over medium heat 2 to 3 minutes to wash down sugar crystals from sides of pan. Uncover and cook, without stirring, until mixture reaches thread stage (230°). Remove from heat, and add marshmallows. Stir until marshmallows melt. Add peppermint oil and pecans, stirring well.
Working rapidly, drop pecan mixture by teaspoonfuls onto waxed paper. Let cool; store in airtight containers.