Getting that dirt up from the skin is tricky, but if you have a vacuum available that works with the less effort. Other than that, the traditional curry curry curry the brush brush brush. Haha You can't really make it go away unless you put a sheet on him.
Veg/olive oil or molasses I would not recommend. Molasses is super high in sugar, which can cause metobolic disorders, high energy, or poor digestion. Sugar is actually not good for horses.
We feed soaked flax seed, which you can buy at your local feed store. You soak one cup flax to 3 cups water. Soak it over night, stir it all together and mix with grain/beet pulp/rice bran, etc. You can alot of it and all of the horses who are on it at our barn are incredibly shiny even in the dead of winter. :)
There are flexible rubber curries that are great for legs.
"They" say that if you groom a horse right, you'll burn 350 calories in one session. So don't give up after 10 min.