Do you have a config file we can add any fluid we want, like the original diesel trains did?
No currently not. the old mod actively went through all existing fluids and could work that way. This mod currently only looks for specific fluids. I assume to add custom fluids you would unpack the mod zip file and edit the config.lua file in there? I could write up something similar, but (especially for multiplayer games) it would actually be easier to just put a simple mod on top of this one that adds a new fluid to the diesel locomotive. I'll write up a guide on that tomorrow.