I've been running a 3M Filtrete MPR 2800 in my furnace filter. https://www.purpleair.com/ confirms it works. There house averages maybe 60 PM 2.5 vs 200 outside.
Don't know how good it is for furnace, but it hardly runs otherwise in Northern CA.
Do you have two air sensors, or do you just have one indoors and compare it with weather data?
I am relying on 2 purple air sensors 10 miles away in opposite directions for an outside estimate.
They are usually within 10%, and we are in the same valley, so should be close.
I also see when I stop the furnace fan, levels spike. Then drops when I turn on again...