Greek yogurt is especially creamy, and, according to George Mondiotis, author of Traditional Greek Cooking (Canada, UK), "probably superior to any other obtainable in Europe." What hope, then, can we have in the relatively yogurt-impoverished United States? Greek varieties made with ewe’s milk contain about 5% milkfat, and cow’s milk yogurts contain 9% (as opposed to whole-milk yogurts in this country which have around 3.5%).
If you substitute plain, American whole-milk yogurt in your recipe, it will fail miserably, and your self-esteem will suffer. Then again, none of the eight Greek cookbooks we checked specify Greek yogurt in recipes that call for yogurt, so we’re guessing most any whole-milk yogurt you find will work.