This link below may be some help - and then grabbing a free account (if they still do that!) on Feed XL and entering every last item your horse eats/you are thinking of adding can be helpful in finding any problems or potential problems with diet.
As for feeding oil, I’ve been doing that for about 30 years to various horses in various amounts from 1/2 - 3 cups daily. I have not had issues with rancidity in summer since going away from Costco bucket or grocery store jug containers. Keep it in closed, dark containers out of sunlight (not by a window - under a table or on a lower shelf, etc) and preferably use a container that doesn’t need the top taken off - think Cardbordeaux or pump jug type dispensers. The only times I’ve had issues with rancidity were when I was using large buckets and took the lids off daily. Even then, it’s about palatability more than making a horse ill. That said, chucking out expensive oil because it tastes bad is frustrating.
If you’re really worried, keep it at home and measure out daily servings into dark, individual sized containers and take those to the barn as frequently as possible.
I am all for adding calories through oil. Like I said - 30 years of adding significant amounts and I don’t see that changing. I like that way I can adjust calories without messing with nutrition , and can very easily adjust seasonally to decrease calories when pasture is good and increase when pasture is nonexistent.
