The following has been tested by guys I race with, as well as LHS personnel. My personal recommendation is to stay with the manufacturer or Novak Synthesized.

You must use crystal sets from the same manufacturer as the radio (i.e. KO radio use KO crystal sets for both tx and rx).

JR radio can use JR, KO, Futaba tuned XXL, or Novak Synthesized. If you do use the XXL in a nitro car, make sure you pad them with foam and shock mount them.