What is the liquid in a Radon U-tube Manometer?
It is oil of a specific density to correspond to the scale. I believe red is one density and blue another, at least for Dwyer manometers. I'm fairly sure replacement oil is available. If you don't care about the accurate reading and only that the fan is working, you could just use plain mineral oil, but you should post a notice next to the gauge that the oil is non-standard and what the last known good reading of the new oil was. I wouldn't use vegetable oil as it may get rancid.