Get her teeth seen to by a *good* equine dentist if you haven't already. It may be that due to tooth loss or damage she can not chew long stemmed fiber such as hay, so would be better fed chaff instead. I also suggest getting onto an equine nutritionist(pref independent of a feed co). I personally use an online service/program called FeedXL.com who I find invaluable and very economical.
Oil is one ingredient you can add, and in reasonable quantities - such as a cup or 2 per day, I haven't seen probs. BUT I know corn is a particularly problematic grain for horses & not sure about corn oil. Basically any other vegie oil is OK, but the cold pressed ones, such as flax/linseed, have a heap of essential fatty acids & nutrients in them too.
But the prob may be what/how much/how frequently you're feeding presently. Firstly, horses don't generally cope well with grain or other high sugar/starch feeds. They can cause hind gut problems & effectively cause weightloss, among other probs. Horses also have very small stomachs and quick metabolisms, designed for eating tiny amounts very frequently. The bigger &/or more infrequent the feeds, the more problematic they can be - especially if feeding sugary/starchy feed - the less is likely to be digested, the more wasted. If you want to continue feeding grain, at least make sure it's well processed and fed little & often - eg at least 4 feeds daily of up to half a scoop with a fair bit of chaff. But there are also healthier, safer alternatives to grain, such as copra, soy, etc. as well as manufactured feeds such as Hygain Zero.