Walnut oil has a distinctive, nutty taste and is used mainly for salads rather than as a cooking medium. You'll find it in many supermarkets and gourmet food stores. For the best flavor, buy roasted walnut oil, made from roasted nuts. The French term for walnut oil, huile de noix, may appear on the label. Because it turns rancid quickly, refrigerate after opening; it will keep for up to 3 months.