Substituting Oil for butter

I’ve not done this often, but… I substitute equal proportions by weight, but I only sub part of the butter. I think I’ve replaced 1/4 of butter; can’t remember for sure at the moment.
However, because oil is liquid (v. butter as solid) it can weigh down the batter. (Butter has water that turns to steam that makes the cake rise, right? Oil doesn’t do that.) So I increase the leavening to account for it. You’ll have to practice to see what works for you.
Good luck!

Here is a great article on replacing fats in cakes :) Hope it helps!
http://www.thecakeblog.com/2012/05/is-butter-better.html

And in case it doesn’t mention it, I have read that you can replace the same amounts for butter and oil. I always do!

Here’s the reply by Summer who wrote the article in the link above:

Thank you for all of your kind and inquisitive comments. I substituted by volume in the experiment. So yes, the amount of fat in each product will vary slightly due to water content and aeration/gas content, but I determined that it would be easiest for readers to implement the results of a direct volume comparison. And I did substitue cup for cup with each of the fats. Also, yes you can easily do a half and half butter and oil cake. Most of the cakes I bake are butter based but use at least some oil. If I try a new recipe and it is a bit dry I usually add oil in, 2 tablespoons at a time (for a 9-inch round cake), until the moisture is correct. The oil I used was canola oil but any mild flavored oil would work. Lastly, we do carry some unsalted margarines here in the states but I have not used them in cake baking. If salt is an issue it could be adjusted from the ingredient list but the texture of the margarine cake was more of the deciding characteristic for me. I hope this helps. Happy baking!

And you cd use canola or sunflower oil. They are both tasteless and light.
R