The Mimeo Ink.
The ink that came with my Print-O-Matic was long gone. The contents of the can had a pretty distinctive smell, so I assume the original ink was oil based.
The only sure thing is that india ink like the one you use for brushes and dip pens is not working - it's just too thin. So my next step is testing some screen/block printing inks. They will probably be closer to the consistency of the original ink.
Also, I got some advice from a person who used those kind of simple mimeographs. They have since gone silent on me, but the advice could still be helpful:
You can use RISO inks, if you need to thin it out use Linseed Oil, nothing else, linseed will thin and not break down the ink. The ink should be semi paste, like consistency of pudding from the store. Or cheap yogurt.
They refer to RISO inks that come in a tube. You can still but them as new old stock, but I want to test some contemporary inks first.
I have also seen acrylic paint mixed into nori paste base used and spread using a squeegee.
More info will follow.