I cannot find any "real" measurements of the airflow through various
filters, 3M vs. Accumulair vs. fiberglass filter, etc. I just find
discussions about "this one being more restrictive than the other" or that
3M Filtrete purple has 15% more airflow than 3M red, and so on. It's all
very unscientific, just people saying whatever they guess based on how much
dirt appears and how much the filter warps toward the fan.

Is there an objective measurement somewhere? For example in some test
setting when starting clean, fiberglass flows A amount of air, 3M purple
flows B amount of air, Accumulair Gold flows C amount of air...