At the risk of being stoned for going against all the forum wisdom, I use a 50/50 mix of 100% acetone (Kroger generic fingerpolish remover is just that) and PGA or 90% plus IPA if PGA isn’t handy. I do not soak the stem, I just run a pipe cleaner moistened in it through the airway. I have used this for 40 years with no ill effects, on one Castello I have owned since new for over 30 years, hundreds and hundreds of times.
A video posted here showed all sorts of instant mayhem wreaked on exposure of “heat sealed” acrylic to alcohol. I have no idea if the airways of Castello stems are heat sealed or not, but my experience, both with routine cleaning of pipes I have owned since new and the grungiest estate pipes, convinces me that what I do is safe. And the formula was developed by the late Joe Zieve of Smokers Haven in Columbus and sold for years in his shop so his customers could maintain the thousands of GBD’s he sold with clear Perspex stems.
But GLP has said otherwise, so forget everything I just said.