Add broth, macaroni, oil, salt and pepper to the Instant Pot and stir.
Put the lid on, turn valve to sealing, and press Manual, high pressure, and select a cook time of 2 minutes. The pressure will build for roughly 7-8 minutes and then start counting down.
When the cook time is over, turn the Instant Pot off and do a controlled quick release, watching for any foaming. If the steam is clear, quick release any remaining pressure (do not do a natural pressure release!).
Turn the Instant Pot to saute, and using the adjust button, select less heat (optional -- the heat of the pasta should melt the cheese if you leave it off)
Add the milk and cheese and stir until just combined. If it's too thick, add a splash more milk. Too thin, add a bit more cheese. Note that it will thicken as it sits.
