To make things even more confusing Schmidt is referred to in a couple of ways. Heres my personal summary to help things for my customers:
Cross refills:
- Used in most slim, etc type kits.
- Considered a ball point ink.
- Not as "wide" of variation in ink quality as other refills overall
- My personal favorite is the Private reserve, but I occasionally find other brands that are about the same quality.
Parker compatible
- Used in Sierra (and similar), Cigar, "pro-click", and probably other twist kits
- Considered a ball point ink.
- The Private reserve 9000 is by FAR the best one that I've found for this type of kit. However, I have found that there is a few people that still prefer the parker gel, and even a smaller number that like the original parker style.
Capped pen compatible:
- Used in Zen, Jr Gent (and similar), full size Gent and similar, baron and a ton of others (which is why I called it "capped pen compatible"
- Considered a roller ball ink
- Often marked 888 or 5888 and often called a Schmidt refill (which is why the PR9000 by schmidt is so confusing)
- From what I understand the difference between the 888 and 5888 is the 888 is plastic bodied, the 5888 is metal bodied. I don' have enough experience to know if there is significant difference... Both write nice to me.
- Because of the rollerball "needs" a capped pen, not a twist due to exposure to the air more (from what I've been told).
Then you can get into FP ink... and that is best left to someone with WAY more info than I have.
(note - this is all gathered from my experiences/learnings and thus I should not be considered an expert
)