11.0 Can I use any fountain pen ink in my pen?

Provided the ink is labeled as "fountain pen ink", you can use it in your pen. But, remember, it has to be labeled "fountain pen ink". If it isn't labeled "fountain pen ink", it will probably cause you some grief: from clogging up the feed to causing damage to internal seals.

India ink is not fountain pen ink! Using India ink in a drawing pen (like a Rapidograph) is perfectly ok. But, don't use it in a fountain pen, unless you want to have a premature experience with nib and section disassembly!

Also keep in mind that all true fountain pen inks are water soluble. If it isn't water soluble, don't even consider using it in your pen.