A close examination of the OHTO gel refill, the PG-805NP, it’s clear that the replacement candidate field will be narrow. Not only does the head of the refill need to have a small enough diameter to pass, it has to be followed by a long narrow shaft. Most refills end up stepped out earlier up. Note that this original refill was renamed to PG-105NP.
But I have managed to find a workable candidate. The Zebra F-Series. The shaft length from the tip to the point where the refill widens is a match. However, the length is a little bit short. No worries–a plastic spacer of the same diameter will do the trick. The tip diameter is just a touch more narrow. Probably enough for a little kapton tape. But the tightness is strong enough that you really don’t feel tip wobble.
Follow up: I have a feeling my F-Series refill is a bit old. The ink is flowing OK but it’s rather mediocre. Based on how it looks, I’m thinking it may be a candidate for conversion to D1 refills. But I’d rather avoid that since the pen body can obviously accommodate a lot more ink. I’m planning to just order some PG-105NP refills. From what I’ve read, this “flash dry” ink is supposed to be decent and difficult to smear.
If any of you guys know of other compatible refills, please post. Thanks!
UPDATE #3:
My order of PG-105NP refills came in. 5 for $7.35 USD from Amazon USA. I can affirm what @drifand said – these are a perfect match. OHTO simply changed the refill model number for some reason. Odd that they don’t include that in the notes for this. Maybe the PG-805NP is old enough that they felt no need to bother. Anyway, this “flash dry” ink is very nice. I expect it will have about a year shelf-life once unsealed. Now I have to find a rubber end-cap to help reduce drying out.