Feeding oil

[Snipper]

My friend has a 19-year-old mare who is about 15.2hh. She is fit and healthy and still hunts weekly during the season. But she does want to get more weight on her. She is on build up nuts, but she wondered if there was something else she could try. I suggested feeding oil might be worth a try. Is this right and if so, what oil should she use and how much? She doesn't have any extra oil in her feed at the moment. I thought that you had to feed quite large amounts but introduce it slowly? If you don't think this would work, what else would you suggest?