Is butter specific to all bread mixes?
oh, I understand your question now. To my surprise, the mixes I examined (King Arthur sourdough and french herb bread) do call for 2T butter or oil. I think it will turn out okay without it, but I'm guessing it will be better with. Any reason you don't want to use it?
If the boxed mix specifically calls for butter, I would use butter. If it calls for melted butter, you should be able to substitute oil without issue. I think butter will yield the best flavor for most breads, though.
Butter will usually make a bread richer and a bit heavier (not in a bad way). I have very good luck substituting oil in its place in bread all the time, but I'm careful to watch the amount of oil. I'll use a bit less oil than it calls for butter to start, sometimes adding more oil or another liquid if the dough seems too dry. Remember that it's a whole different ball game if you're cutting the butter into flour. Substituting oil only works if you're creaming or melting it in.