Which weight supplement for my horse?

I own a 16 year old Trakehner gelding. He's not an extremely hard keeper, but he's always on a weight supplement for maintenance. Right now I have him on Platform Weight Builder, which works wonderfully, but is kind of pricey. When I first bought him, he was quite thin for his size, so the vet who did the pre-purchase said to give him up to 4 cups of veggie oil per day. I would like to switch him to something a little more cost effective, but most of the other weight builders are mostly corn. I am open to using the oil again, but I've heard CocoSoya is amazing. Is CocoSoya all that it is hyped up to be? Suggestions of any sort would be greatly appreciated. I'm open to anything that works!