I use baby oil on my mare's mane and tail dock, and also the hair to make it easier to de-tangle. I find that you don't necessarily have to wash it out, like MTG, and when it soaks into the hair it seems to make it less brittle. My mare's mane is down past her shoulder and I just trim the ends when they get to straggly, keep it in braids and only re-braid it if the braid looks like it's trying to mat.
As for her tail, I had it all the way down to the ground and thick and gorgeous, and then one day I went out and found that her tail was all thin and ragged and I freaked. Looking more closely I found that someone had cut about a 6 inch wide chunk out of the underside of her tail, AT THE DOCK. it was less than an inch long. I was crushed and freaking out as to who the hell cut my horse's tail off for no good reason. Later, I come to find out that it was my sister-in-law with my mother's help and they took the hair and had a really neat bracelet made of my horse's hair, so that she's always with me. Sort of stopped me freaking out and the chunk that was cut out is past her hocks now, almost looks normal but when it started re-growing it was all bushy at the top and looked thin at the bottom lol.
The point is, it's just hair. It'll grow back. No need to freak :)