You can use a detergent but, make sure it does not have fabric softeners or other useless chemicals in it. These things coat the fibers and tend to make your towels less absorbant. You will want to stick with a liquid detergent as it is possible, albeit remotely a powder might find its way to your paint surface and it will scratch just like sand.
You might also want to consider microfiber. I get mine from www.pakshak.com but, there are other vendors out there. If you consider buying microfiber get it from a reputable source as quality control is an issue with some of these vendors that buy from whomever came in with the lowest bid. You can find more info at www.autopia.org, just do a search for microfiber.
Read up about towels on the Zaino website. There is a ton of info on towels there. Zaino recommends that you use a liquid detergent since power can remain in the towel and scratch the paint.