Oils are 100 percent fat, so they must be reduced by 15 percent to 20 percent when substituted for butter, either by weight or by volume measure. However, there are additional complications when substituting them for solid fats, especially in baking. Since oil does not contribute flavor the way butter does, cakes made with oils need lots of flavor from other ingredients, such as tangy citrus juice and zest spices, chocolate or cocoa, and/or toasted nuts.