What you need to know: using melted butter rather than oil will probably improve your cake. I use butter in everything I bake, rather than shortening, margarine, or other oil.
You won't get good results substituting butter for oil. Your cake will be lumpy and heavy. It is better to substitute applesauce or banana. Adding just a tablespoon of oil will get better results but is not necessary.
For cake mixes, I substitute the same amount of melted butter for oil, then I proceed as normal. I have found no need to change baking times and temperatures.
