The official position of the company can be summarized as follows:
1. Pilot’s Parallel Pens use a dedicated ink—more fluid than most standard fountain pen inks. And all of those dedicated inks are mixable among them.
2. The cartridges containing that ink are the same as the regular Pilot cartridges. However, inside them there is a metallic ball.
3. The converter included in the package is only for cleaning purposes.
My experience, as can be seen on the video, is different:
1. Pilot’s Parallel Pen inks are indeed more fluid than other inks, but I have used some other inks –by Sailor, for instance—with no mayor problem. Slowing the writing pace avoids any problem associated to the thicker texture of those inks. And we all know that most inks are mixable—Sailor’s, in particular, are well behaved.
2. The cleaning converter can be used as regular converter on these and on other Pilot pens.
From left to right, standard (empty) Pilot cartridge, and converters CON-20, CON-50, CON-70, and CON-W. The CON-W cannot be used in the Parallel Pens.
3. Standard CON-20 and CON-50 converters can be used on these Parallel Pens. The CON-70, with bigger ink capacity, does not fit inside the barrel.
4. There is no problem in using these pens as eyedroppers, thus enlarging their ink capacity a great deal.
In my opinion, the only point of those warnings, as stated by Pilot, is to protect its market of dedicated ink and cartridges.