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.

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.

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?