I have the polyester/nylon/spandex underclothing and I've used Woolite for sink washes. Is there a better detergent for this type of clothing? Is there anything you can use (or add to the Woolite) that will make the underclothing less odor-prone? I was thinking you might add a little anti-bacterial soap to the mix.

Synthetics are stinky. That said, I'm not sure if you are talking long johns/ long underwear. If you are, I would suggest investing in a nice merino wool baselayer shirt. They're great. Wore mine on a five day backpack and no stink.
If I do find myself "sink washing" I have usually used Dr. Bronner's soap-peppermint scent. It can be used for everything, dishes, body, clothes....good for camping and traveling.

I survived the 60's but I cannot stand Dr. Bronner's soap. Instead, I use Camp Suds.

I also suggest that stinky travelers buy products that utilize Visa Endurance. It works.

Thanks for the tips. I've found synthetics to be less stinky than expected, but I'm always looking for a better deal. To be frank, I'm cheap: thus far going with $10 Champion briefs and undershirts from Target and $6-$9 closeouts from Campmor. And BTW, thanks to your tip I've found a number of Terramar products with VisaEndurance at Campmor - thanks!

Of my synthetic undergarments, the mid-weight Capilene shirts catch odor quite fast due to the narrow cut at the armpits. Wearing a tight-fitting cotton T-shirt underneath the Capilene solved the problem. I try to use older clothes during sweaty hikes/walks and have ready a clean shirt upon arrival.

I've gone strictly silk for underthings while traveling. They wear well and are lighter and easier to pack than most or the synthetics. They also clean better and dry very fast. And yes I use the travel size woolite.

I'm going to do a little experiment with Woolite - adding some anti-bacterial soap and/or Febreze to see if it makes the polyester and nylon garments less odor-prone. I'll post the results.

I can further report that a mix of a couple of glugs of clear anti-bacterial hand soap and a spritz or two of Febreze anti-microbial spray in a bathroom-sized sink of lukewarm water seems to make my polyester/nylon briefs and undershirts less odor-prone. I don't add it to Woolite or Camp Suds - the hand soap is the detergent. I'd be interested to hear if it works for anyone else. I also acquired some Terramar briefs with Visaendurance fabric (on the advice of Static) and they are great.