Rice bran oil is a nice addition. I would start with just 1/4 cup per day. Fortified Wheat Germ oil also works nicely, and is usually less expensive.
Too much oil, or any corn oil, can be bad for nutrient absorption. Corn oil can also increase inflammation in arthritic horses. However, products like flax, rice bran, or black oil sunflower seeds ALL contain
oil, so a small amounts of the right kind of oil are just fine and can be beneficial. Good flax is up to 50% oil in its milled form