I have used Wool mix or Lux, either liquid or flakes. If using flakes make sure they are dissolved.
Now I just use whatever I have and rinse with fabric softener then spin in a pillowcase.
The main thing is to make sure the soap is removed.
Hang on a stocking threaded through the sleeves and pull the stocking tight and peg the end and maybe middle of stocking. This will leave no peg marks on the jumper.
Hope this helps.