Honestly, I just sub 1:1, but this site which is very reputable disagrees. Here's some info. Hope it helps!

Here is a link that might be useful: baking 911

Cake mixing methods:

-Shortened cakes utilize solid shortening (fat/sugar creaming method).

-Unshortened cakes (sponge or foam cakes) are made with beaten egg whites for air/leavening.

-Chiffon cakes are a hybrid of shortened and unshortened cakes where the fat is usually from vegetable oil and egg yolks and is combined with the foamed egg whites, cake flour, and leavening agents.

In cakes, solid fats contribute to tenderness, volume, moisture, and flavor. These attributes are distinctly associated with solid fats because vegetable oil doesn't entrap air during creaming. Using vegetable oil where butter is called for will result in decreased volume and a "harsh" crumb. As with all cooking/baking rules, there is ALWAYS some exception, and this exception is olive oil, because it contains natural emulsifiers to make cakes more tender and moist. You may have better luck using a light olive oil.

Vegetable oil coats the flour proteins preventing them from adhearing to water. This reduces the gluten formation and leaves more water in the batter (gluten absorbs hydration). So if you use oil in a recipe that calls for butter, I wouldn't use it 1:1. I'd use less oil.

The key... not to add too much oil or the cake becomes too heavy and compact (I'm thinking of my MILs OILY 5-pound pumpkin bread recipe LOL - while my favorite banana bread recipe only uses 3 T. of oil - but I digress).

Air bubbles aren't easily incorporated into oil as they are butter. You are now stuck with only the leavening agent remaining to create air bubbles by using oil instead of butter - the chemical leaveners (soda/baking powder) - OR - physical leavening from whipping air into the batter (via egg whites). So if you substitute oil for butter, you may want to whip the egg whites and add them to the batter to aid in adding air bubbles to the batter.

When making muffins, the traditional muffin-making method the steps are geared to minimize mixing so that you don't develop gluten. In traditional muffins, the fat is most often in liquid form - both melted solid fats or liquid fats (cooking oil). This is mixed with the other wet ingredients and then added all at once to the dry ingredients.

In a muffin recipe where the fat is solid and you use traditional cake methods - creaming the fat with the sugar - the resulting muffin will have a more cake-like texture.

HOW fat is incorporated will create different textures, and all the time the fat plays a BIG roll in minimizing gluten development. You can also aid gluten development up front by using low-gluten flours - something lower in gluten than all-purpose flour - a southern all-purpose like White Lily or Martha White will make the crumb more tender than all-purpose flour. To off-set the additional gluten in all-purpose flour (say if you use a higher-gluten all-purpose flour like King Arthur or Hudson Cream - a Kansas brand of flour) is to add a little more fat so you don't get a tough crumb.

How much you beat the batter will also alter the texture. I NEVER use an electric mixer when adding the liquid ingredients to the dry ingredients. I use a Danish Dough Whisk. Keep the mixer for creaming fat/sugar.

Different mixing methods will get your different kinds of cakes. What you are trying to get done is to incorporate all the ingredients as well as the maximum number of air cells - which you need for vulume and texture. Some pitfalls...the temperature of the butter, eggs, and the mixing bowl; insufficient creaming; insufficient blending of the baking powder and flour; developing too much gluten/using flour that is too high in gluten....

Cake is a ratio of ingredients one to another, mess with the ratio by altering one or two ingredients and you have to adjust it someplace else to compensate. If you are working with "high-ratio" cakes that contain more sugar than flour, then you have a whole different set of "rules" to consider. In her book, "Baking 9-1-1", Sarah Phillips mentions making a cake recipe over 100 times until she made a reduced-fat version of it she was satisfied with.

-Grainlady

Use 3/4 cup of oil in place of 1 cup of butter.
You need to increase the sugar too, though, and I don't know how much. (I always go in the other direction; I made up a carrot cake that used butter.) Because oil doesn't have the nice taste that butter does, oil cakes always have more sugar to cover that up. Look around on the web for quantities.

Thanks all for your responses. And especially to Grainlady for your lengthy explanation - I am clearly out of my element here, that's for sure. I ended up reducing the oil by 1/4 and the recipe DID already call for whipping up an egg white, which I did. But, although the result is getting a little closer to my goal, it's still far away. As predicted by Grainlady, it was a heavier texture than I want, less cake-like and more like heavy banana bread. Perhaps I need to increase with another beaten egg white? Maybe White Lily or Martha White flours you mentioned are like cake flour? Or, should I just try cake flour? It's all so confusing when one knows zero about baking, other than following a recipe. I mixed the brown sugar and oil 1st, added an egg yolk, then sifted the dry ingredients together (AP flour, salt, 1-1/2 tsp. of baking powder) and added to sugar mixture, alternating with buttermilk in three turns, mixing by spatula only. Then, folded/turned in the egg white. This was for a 6-cupcake/muffin attempt (I halved the orig. recipe). I'm already SICK of them, lol.

arline

I've heard 7/8 cup oil for every cup of oil.... also I always use whole wheat flour and a trick I've learned is to put 2 tablespoons raw apple cider vinegar into the recipe. Not sure how it works but it always makes the texture smoother and lighter. White vinegar might work as well. Good luck.

I was interested also how to make the substitute for butter to oil because I had heard it produces a moister cake. Confused now after reading all of the above and wondering what I am gaining by doing so. Anybody know???