Yes, you can always use instant. But food editor Margo True swears by steel-cut: "I love it because it has a chewier texture than instant or ordinary rolled oats and a deep, nutty flavor." It also freezes well (add the fruit after defrosting), so make a big batch on the weekend.
My family enjoys this dessert anytime of the year, but it's especially refreshing during the hot months. You can use whatever fruits are in season. —Ida Ruth Wenger, Harrisonburg, Virginia