Try the dealer for citric acid. The part number is 000 989 10 25. I've never used it. I don't know if it comes with mixing instructions. The manual says 100g/l.

I've read of folks using dishwashing soap as a de-oiler. I doubt there's anything in dishwashing soap that's harmfull to the innards of a 617 even at full strength.

I've only used 2-3 flushes with tap water. It comes out clean enough that I don't feel the need to do more.

Sixto
91 300SE
87 300SDL
83 300SD