just pop yer peeper in the hole on the STP'S some are E-cores, some are not.
only 2 i've bought were a deal @ autozone last year (5qts Mobil Super 5000 & STP filter for $9.99)
both got hacked up unused, for practice @ filter cutting.
the Neon's filter was an e-core. down to the plastic cage in lieu of the center tube.
the vibe's filter was well made. metal in all the right places.