dont get the $1 stuff and the $20 ones are overkill. and the reusables
ones are very unpractical unless you like the hasssle of wasting time
and energy washing a nasty spong once every couple of months.

the $5-10 filters clean the the air very well and the V shaped ribs
allow more surface area vs. the flat $1 filters so they flow much
better.

change them when you visualy see dust on the surface. change intervalls
really depends on usage. the 90day vs. 30day spec is really nothing but
BS.