I have used Argan oil in the past.
You can also use mane and tail oil bases conditioners--Healthy Hair (aka Miracle Groom) is great, the oil is good for moisturizing but if you follow the instructions for the dilution, it doesn't leave your horse greasy. You can also use it all over your horse :)
careful with oil if you turn your horse out in the sun. Just like baby oil on skin, it can cause sun burn/bleaching.