I want to add a custom fluid. How can I do that?

If it is a diesel implementation of a well known mod, ask me to add it in for you. In order to avoid too many individual requests I will choose which fluids I will add in and which i will not.

If you just want a new fluid for yourself (or you and your friends), you can simply write a small mod (only 3 files) that adds additional fluids to this one. And feel free to publish that mod as well. Here's a short guide on how such a mod could be built:
https://github.com/ksmonkey123/factorio_dieselTrains/wiki/Adding-custom-fluids