I have this great recipe for a white cake. The volume, taste everything is perfect except it could stand to be a bit more moist. My question is could I add oil or some other ingredient without throwing the whole thing out of whack? It uses cake flour, egg whites, butter, sugar and extracts. Any suggestions?

I would add sour cream it makes cakes more moist.

I do all the time ... Pudding .. sour cream

I like to add mayonaise to mine. I add about 1/2 cup of miracle whip per batch. The egg and oil add moisture, and the vinager acts as a natural preservative.

I've never met a recipe I couldn't tweak. In fact, as I'm reading a recipe I'm thinking, "oh! oh! I know! I could add some . . . .to this and really kick it up!"

I usually (though not always) make the recipe as its written the first time, then tweak it one ingredient at a time till its perfect for my purposes.

To moisten a dry recipe you could add oil, but I wouldn't (you might end up with something heavy and greasy). I'd add sour cream, mayonnaise, or unsweetened pureed fruit. Unsweetened applesauce is a good choice because it tends not to change the flavor, but I've used mango or crushed pineapple to good effect as well. I would start with just a quarter cup of whatever you're adding and go up from there. I would do the addition with whatever liquid the recipe calls for.

i often add yogurt - it won't add fat content to make it heavy but adds wonderful favor and moistness.

I've only used plain yogurt and yes, I replace sour cream with yogurt almost always to cut fat.