Honestly if you just want a shiny coat and a bit of weight, I’d just get some canola oil and call it good. Its O6FA:O3FA ratio is 2:1.
No shame in adding extra calories, especially when you have one that is tougher to keep weight. You can safely add up to 2c/d of oil to a ration and would be just straight calories. I would honestly try that before any of the more expensive supplements unless you are specifically looking to add EPA/DHA to the diet (and in that case, you need a O3FA supplement that is fish oil based). My older horse is a bit tough in the winter, so on top of a high fat feed, he gets a big glug twice a day. He is one that in the summer he will look like a round WB if I am not careful, so I have change his diet seasonally (he is 19 so I tend to keep him on the lower end of ideal BC during the event season).
My younger guy is an easier keeper, but he gets oil as well because it makes his coat super shiny.