using it as a sauce for a stir fry.
The main difference would be the salt content of the two soy sauces -- dark soy sauce or tamari is much saltier spoonful-per-spoonful than regular soy sauce. You might try substituting half dark soy sauce and half water.
Dark is aged longer, it's primary used for 'red cooked' meats. It has a deeper flavor, slightly thicker and has molasses added for the aging...and some say it's less salty.
Light (Not that abomination "lite") soy sauce will just be fine in a stir fry.
Not to thread jack. But some soy sauce expert that sees this thread might be able to answer this one:
I have a half used tin of Kikkoman's Soy Sauce--that's been hiding in the back of a bottom cabinet for at least 5 years. I opened it up and poured out a sample. Whoo! It's fermented into a alcohol smell, and taste pretty strong. Not overly salty tho..just a heady alcohol wiff. I hate to throw it out because it's so unique.
Did I inadvertently make 'dark soy' out of that...or some type of soysauce liquor?
the host of a PBS weekly series on all things food did a blind taste testing of soy sauce and rated Kikomen's the best. oh the show is called A Splendid Table-