We always fed raw whole linseed to the horses when I was in Germany. It was quiet a surprise to me when I came over to the UK. However No-one believed me!
I won't comment on the feeding v washing advice, but I do sympathise as my dun native has a lovely shiny coat, but lives out. Mud and dust will dull any tail when it's attached to a dirt loving pony!
When necessary I wash it with Red Rum coconut shampoo. Fab shine and feels silky. All I need.