Borax and hot water?
I wash most of my untreated and not outer wear synthetics with hot water, a good soap, and borax. Usually helps. I also oftentimes after the wash and rinse cycle, put a little pre-dissolved water/borax solution in towards the end.
Seems to temporarily help with synthetic stink issues, but it's not good for outerwear type stuff that will be used for rain protection or the like, since it tends to decrease the hydrophobicity of the synthetics some (leaves a water attracting residue i think).
Which made me realize something recently. When i talked about my cheap nylon microfiber boxers a little while ago on my thread here asking about good wicking nylon shirts, well i recently washed them without using the borax treatment and no they do not wick as good as i thought they did. Previously when i tested them, they had been "treated" to the above borax solution, which is apparently what increased the wicking action. Felt i should correct that.