Ny level of radon in the home that is *immediately* dangerous

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.

Fresh air from outdoors is presumed to be lower in radon concentration than air that is trapped in the house. Any introduction of outside air will thus lower the radon concentration inside the house.

The answer to your exact question depends on your goal:

Are you trying to see the maximum possible concentration in your house? Then you should keep all windows and doors strictly sealed.

Are you trying to see the concentration you experience during normal life in your house? Then open windows and doors as you normally would.

Are you trying to get out of doing mitigation? Then open everything and let the fresh air circulate.

Note that radon concentration changes with, probably among other things, the height of the water table underneath the building. In my house, this results in a seasonal variation that a short-term test wouldn't catch. I determined this with a yearlong test using an electronic radon meter.