Check your feed label. You will want to at around 5%-6% fat in the ration. Vegetable oil is around 7.5 lbs to the gallon. If you feed 24 lbs. of ration to your steer, and if there is no fat in the ration, you would need to add 1.44 lbs of fat or around 24 oz. of liquid (if my math is right). If there is already some fat in the ration, then you will have to figure out whether to add fat and how much.
I have been feeding a mixed grain ration (that's all it is) with some cottonseed hulls mixed in for texture. We feed about 2% of body weight of the grain ration and they seem to gain fast enough. With a 750 lb. steer, he will get 15 lbs. of grain and 0.75 lbs of oil, which works out to a tenth of a gallon of oil, or 12.8 fluid ounces, i.e. a little over a cup and a half of oil per day mixed into the ration.