I think it would be useful to distinguish right and left mouse button in fluid io. RMB would be responsible for inserting and LMB for extracting fluid from machine (or vise versa, idk). This would be useful when working with larger tank (for example those from mekanism, including gauge dropper, which takes only 125mb per operation). Yes, it's already possible to fill entire tank using shift, but it's not as convenient as it could be.