What is inside an oil filler/breather cap?

DFX 340 Duster said
You could try using methanol (methyl alcohol) it will loosen debris, remove varnish build up, not destroy the filter media, and it will dry without needing a water rinse.
Alternatively you could use isopropyl alcohol (rubbing alcohol is too dilute) but it takes a bigger amount to do the same job as methanol.

You could shorten they drying time by using gentle compressed air.

I wound up using denatured alcohol. It really wasn’t very dirty. The only thing that came out were orange paint chips I guess from the oil cap scratching the valve cover paint. I poured some engine oil inside but didn’t get the feeling there was any kind of filtering material in there.