In a saucepan, combine catsup, vinegar, oil, Worcestershire sauce, brown sugar, mustard, and garlic. Squeeze lemon juice into sauce and add 1 lemon half shell. Heat slowly for about 10 minutes. Sauce does not have to reach the boil, but heating blends flavor. Use sparingly as a basting sauce for fresh pork, ham, ribs, or chicken. Heat additional sauce and serve as a table sauce. Makes about 3 cups.