hot oil is a godsend. I have a brown horse with a black tail. I always used to look enviously at the other people at shows who had those mirror-shiny black tails, and no matter how I tried I could NOT get it to happen. Clean, yes. Shiny? Nope! I used show sheen, Mr. Preen, anything I could try.
Then hot oil came along and wow-ee!
I don't know how others do it but this is my way. I shampoo'd the tail, realllly thoroughly. Right down to the dock and scrubbing it. Wash that out, then conditioner, again right to the dock. Leave in for a few minutes and wash out, really thoroughly.
Then I either mixed the oil with a little water in a bucket or in a spray bottle. Not too much oil as if there is too much, dirt will stick. I just sprayed/sponged it all the way through, right to the dock, and finger combed it all the way through the tail. I then let my horse graze while it dries and I finger pick all the knots out. once dry, I plait it and put a tail bag on for the night.
In the morning, unplait last, brush through and give another spray with the oil/water mixture.
I also use the oil/water mix as a grooming spray on the coat.