Koh-I-Noor Ultradraw Pigment-Based Ink, 8 Oz. Bottle, Black

Ok OP, I just went through this with the same pens. Get some distilled water and the cleaning kit with rapidoeze and the red hand pump https://a.co/d/0aOYBhfa , break the pens down just before you get to the wire, and soak them in distilled water and rapidoeze for a day or so and then rinse until you no longer see black ink coming out of them. Then you want to use this ink Koh-I-Noor Ultradraw... https://www.amazon.com/dp/B0768NPQSW?ref=ppx_pop_mob_ap_share, specifically made for these and at this size and cost is the best value for these. These are excellent pens and with a bit of TLC it can last you decades.