Substituting butter for oil: Does it matter for baked goods?
Butter is at most 82% oil and the rest is water. So, you're going to need to add more butter for the same amount of oil. Besides, you need to adjust the liquid content accordingly since you are going to have more water than you planned.
If you are going to use the same amount of oil, then it will not make any difference other than the taste. Just make sure they are of same state (i.e. melted butter instead of vegetable oil or vice versa)
In practice, for most recipes, the water content in butter is small enough that the variance is well within the tolerance of the recipe; in practice, oil and butter can be substituted 1:1 in quick breads like muffins.
Also, mind that butter/shortening/margarine, unless thorougly emulsified, will re-solidify at room temperature wherever it is in the finished food while oil won't. Also, dough consistency during early stages of baking can be different enough to matter, esp if the dough is allowed to chill before baking.
If you're making something like muffins, then in my experience the water content is a non-issue. Yes, there's some water. But muffin batter has such a broad range of tolerance that as long as you're basically familiar with the target consistency it'll be fine.
That said, the amount of oil or butter that goes into a batch of muffins is not really enough to impart much butter flavor.
If I am substituting butter for oil, I like to do a mixture of 1/2 butter and half shortening, this allows the richness of the butter as well as keeping the consistency of the recipe. I also add clear butter extract (flavoring) to the recipe. I use about 1/2 tsp butter extract per 1/2c of shortening and cream the butter/shortening mixture with eggs, sugar and vanilla first before I add any other ingredients. I also substitute shortening for butter sometimes, only I also add 1tbs of water per 1/2c shortening along with the extract. Hope this helps!