I love hand dyeing fabric! I have never used synthrapol - I read somewhere that Dove hand dishwashing liquid works. Since Dove is easier to find, and less expensive, I have always used it and never had a problem. I started using the instruction from this site,
http://fabricdyeing101.blogspot.com/ . They were free (& comprehensive) when I used them. The free directions are no longer ther but now she has an e-book for sale.