I've just got my first dip pen and bought some Diamine India ink at the same time. Whilst I know I've got plenty to learn about writing with a dip pen I did find the India ink very hard to use. It's so thick that the flow sometimes slows and then if I dip again to ease this I can easily get a blob.
At the same time I bought some Diamine Registrars ink to try in my fountain pens. I tried this with the dip pen and it worked great! Nice even flow, and the colour is a nice deep even black because the dip pen writes so wet. Also before the nib runs out of ink the colour of the line goes more blue, to warn you you need to dip again soon. Yet once dry it's the same black tone as the rest. Also I was able to write more letters with the Registrars ink between dips, than with the India ink.
What do other people use with dip pens, any others prefer iron gall to India or drawing inks?