Oil.. Like canola oil? Why do you want to feed oil if your horses are round? DrumRunner posted a great article about oils Adding Oils to Your Horse's Feed.
Oils gain weight on horses due to all the fat, and I don't get why you would need to make sure your horses get their oil if they are chubby? Horses really don't need grain or oil, or anything else when they are fat besides their roughage (assuming it has all the appropriate vitamins and minerals, if it doesn't ration balancers are great). They only need grain or anything fatty if they have trouble maintaining weight, need to gain weight, or need more energy for their type of work.. If the pasture is high quality you don't need to feed ration balancers, its a good idea to get it tested to see if it is deficient in anything. You can also consult your local vet for any minerals that your area may be deficient in (most of US doesn't have enough selenium). Good luck!