Gosh as someone with a hard keeper, I don't know what I'd do without the calorie content of oil! I will say I saw firsthand that corn oil did cause inflammation and discomfort in my gelding who has hock arthritis. He was much worse on it.
I switched to Cocosoya (after trying Canola and RB oil - both of which he would not touch). He is really thriving on it. His coat has never been shinier or slicker - and he had a dull, Appy coat. He just gleams now when the sun hits his coat. It's pricey but worth every penny to us.
We also use Legends Omega Plus - a rice bran/flax pellet for calories. 1800 cals per pound.