Anyone seen those things on some newer autos, heavy trucks, etc? Perhaps the vac gauges everyone is talking about, I have never taken a close look at them. They have some colored plastic in them and say when your filter should be changed, vehicle specific I assume. They seem to be pretty inexpensive looking (hah) and could probably easily be added to a stock air cleaner setup if someone was truely afriad of clean, easy flowing air problems.
When mine (air cleaner) changes color...I replace it. SImple enough.
(shug)