The oil is nice, but expensive. It would be cheaper to use plain rice bran or rice bran pellets. I have had good luck with milled or ground flax. My local feed store has 50 lb bags of ground brown flax for only $28. You would feed 1-2 cups a day for weight gain. You could feed up to 3 cups.
Horsetech.com has NutraFlax, which is a human grade golden flax with added clacium. It's good stuff, but I feed 8 horses, so I had to cut back
. Their prices include shipping.