Cocktail enthusiast Martin Doudoroff explains how to make an Old Fashioned without using any of the “various bad ideas” (e.g. “There is no slice of orange in an Old Fashioned”) that have crept in over the years.
Sugar (and the scant water it is dissolved in) mellows the spirit of the drink. Not much is required, just a little, as the quality of today’s spirits is so much higher than it typically was when the Old Fashioned was born. A little splash of simple syrup generally suffices. Gum syrup, rich simple syrup, demerara syrup, brown sugar syrup, sugar cane syrup (the variety filtered of molasses solids) all are great choices. Agave syrup or other neutral diet-sensitive sweeteners may suffice.
Honey, maple syrup, molasses or other strongly-flavored sweeteners do not belong in an Old Fashioned, which is not to say you cannot or should not create nice variations on the Old Fashioned with them.