Uni One P vs Uni One vs Zento

Hey,

I have always been Pilot C4 guy. But now, I tried Uni One P 0.38 mm, short chubby one, and fell in love immediately. I couldn't find any colours and I did one colours in Uni One, long ones.

I have couple of questions. Zento doesn't come in 0.38, right? Is there any Uni 0.38 aside from One? If not, that's also good, it makes distinction easy.

Why One and Zento so different? I tried Zento 0.5, and it doesn't feel a feeling like that. Is the ink technology different?

If so, how to search for One and One P refills so I get the same quality? I am in EU and mostly use Amazon de, so if you can give link that would be amazing.