I use vegetable/corn oil (whichever one I have a coupon for that week
), two cups a day, one cup per meal
I love what it does to the coat (soaks through the skin and saturates it... like when you eat a greasy potato chip and your skin feels greasier. It's kind of like that effect, only not as unhealthy), and its a cheap way to put on more weight without more pounds of grain.
However you shouldn't use it if your horse is swelling. Indie got a rope burn on her pasturn, and her leg swelled up a bit so I had to take her off the vegetable oil since it can actually encourage swelling
Put her on extra ACV to mix her feed with until the burn is all healed up and the swelling went right back down. Just a cautionary add-in. Otherwise, I love my vegetable and corn oils :)