It is definantly all diet related. My horse was as dull as they get and once we changed his feed...he shines like a mirror. We switched from regular sweetfeed to a pelleted low starch, high fat feed. Here he is pretty dirty still
I do use canola oil. I don't measure it...put a good splash in it as well. I also give her some sweet feed..i had some LMF that you feed only 1 lb a day but she coliced and I'm scared to put her on it again. It's like $25 a bag. I give her alfalfa and she is eating some grass but need to build her up slowly.
I even have a shine block that I use. She doesn't shed very much. She is prone to sunburn as well so I have to sunblock on her face.
Here is a picture of her in her blanket and another one when she wasn't shedding what hair she had.
The last one she had a bath and this is pretty much what she looks like but she has lost some weight...she had a fat neck as you can see.
I used to use chemical sprays to make my horse shine. After this, his coat dulled more and more and I needed to shampoo him more and more. It was a vicious cycle. Then I met someone who never shampoos his horse and shows in a regular basis. His black mare was shiny and beautiful. He also did not use any sprays.
Here's what he told me and that's what I do ever since:
-curry all over with rubber curry to bring up dirt
-use 'flicking' brush to bring up dirt from the skin to the top of the hair and distribute oils (that's where the shine comes from)
-use coat brush to brush off the dirt that you brought up with the flicking brush, clean the face with it
-use a soft brush all over the horse to remove fine dust
-use a cloth diaper or car wash mitten (lamb skin) to get the horse to really shine
The most important thing: you can't get these results with synthetic brushes. As long as you use synthetic brushes, you need to use these chemical sprays and shampoo a lot.
It's hard to make a white coat shiny, mostly because there are more hairs in the white portion of the coat, so it doesn't lay down as neatly as the colored part. I would agree with the diet suggestions. There are many things you can use, but I would add rice bran or wheat germ oil. There are many commercial products, such as Nu-Image or the color enhancers made by Cheval International. These work very well, but would probably cost more than some other things.
The only other thing I could recommend is daily grooming (twice a day is better) if you really want a shine. Nothing beats elbow grease.
Also, besides diet and coat types, I have found a lot of horses with duller coats contributing to poor circulation and tight muscles. Whenever I bring in a horse for physical rehab, their diet doesn't have to change to have their coat shine up nicely as muscles loosen and stretch. I also do not groom as often as some of the owners that bring them in. Grooming is important, but the best way to a shiny coat is from the inside out, and not just what you put into the body, but how you put the body to use.