I'm pretty much strictly a fruit flavors type, so other than banana don't have any thicker/darker juices. Having said that, no problems so far with Cherry, Blueberry, Grape, or Fruit Punch (all mix-your-own, 80/20 + sweetener). I use V4L metal-endcap 6ml tanks.
Just got the Grape and Blueberry, so can't comment on long term usage, but have had the others for a little bit and no problems there...