butter vs. oil in baking

I know this is an old post but for others looking for answers, I found out that to substitute oil for butter or vice versa, you need a ratio of 3/4 C oil for 1 C butter.

I agree with boulangere (and after all, she is the baker!) about overbaking. It could be that your oven temperature is slightly off to the high side. Butter and oil impart completely different qualities to a cake. The much-discussed Food52 chocolate bundt cake http://www.food52.com/recipes/2835_chocolate_bundt_cake is made with oil. It has a very open crumb and is very moist--almost like a steamed pudding. Cakes with butter have a much finer close-together crumb, as well as that all-important butter flavor. Rather than add oil (or applesauce) to a recipe that calls for butter, try checking your cake 5-10 minutes before the prescribed time, and remove it as soon as the tester comes out dry. I bet you'll be happier with teh result.

If your cakes seem dry, you may be overbaking them a bit. Oil works very well in quick breads, and is also used in chiffon cakes (made with separated eggs, and the whites beaten separately and folded in). It won't work in cakes where butter is called for because of the emulsion that is created when butter and sugar are creamed together, and the eggs added one at a time.

I'm not sure that oil vs. butter makes a huge difference with moistness - I mostly notice a difference with flavor! I do know that adding applesauce or other fruit to a recipe helps to make it really moist - also that apple sauce etc. can be used in place of all or part of the oil in certain recipes. Not sure if that helps at all! (Personally - I usually opt for butter...)

It definitely depends on your recipe; I've made cakes with just butter and they are extremely moist. Butter is a fat that can be aerated when creamed, oil can't. You'll miss out on the texture butter offers, but oil is great in quick breads like banana breads (much denser products, not as airy as cake)