According to several manufacturers of equipment, the problem arises on mixing of the oils. They claim the oils if mixed wil cause a tar like substance to form inside the piping. this can clog your expansion device, filter drier, etc. If the piping is reused it should be flushed with the rx11 flush kit to remove as much resdiual oil as possible. If the pipinfg is okay, it is then safe to reuse if necessary, but always better to change it if possible.