A good diet, and a tooth check to make sure the horse can process said diet, is your first and best course of action.
But I swear by Healthy Haircare Moisturizer for a topical. It's a pink, liquid concentrate that you dilute with water in a spray bottle. I put it on manes & tails everytime I groom (focusing on rubbing it into the roots) and periodically on full bodies to keep everything soft and shiny. No drying silicones or alcohols, nor potential slippery saddle misshaps, like so many shine sprays (such as Show Sheen.)