Flank Steak Marinade - 1 cup soy sauce,  1/4 cup packed light-brown sugar,  3 tablespoons cider vinegar,  1 tablespoon Worcestershire sauce,  2 teaspoons Dijon mustard, 1 teaspoon red-pepper flakes  1/4 teaspoon ground pepper,  1 flank steak (about 2 pounds),  vegetable oil