My yearling thoroughbred stud has had this issue since the day I got him. He was very sick as a foal and weanling and the people I bought him from I threatened to call the ASPCA on a few times because of his health and the condition of the rest of their horses ( I ended up buying him because I didn't want to leave him). He is now healthy and looks absolutely beautiful other than one issue. His mane is probably the worst. He has a very oily mane and tail and all down his body where ever you run your hands you get a decent coat of dandruff. He's had this issue since I bought him and got him healthy again. Right now he's on a quarter of a scoop of Buckeye Growth formula since my mom's Friesian/Oldenburg filly is on it (and I just started giving him it to keep his weight up for the winter) he also gets a full scoop of sweet feed to go with it. His weight is and body condition are perfect for his size since he is a rather large baby at 15.1 hh at a yearling. He is brushed better than most professional horses on a daily basis in an attempt to help this problem but it's not making much of a difference.
Any info or recommendations for supplements to help with this would be appreciated.