Iridiums are a newer "best/better" plug. GM puts them in most of their "100K before a tune up" cars.
Like my 00 truck. I didn't touch them til 100K and the gap was ridiculous, at the least they should be pulled and regapped.
I put them into my cars and was unsure, I thought the 97SC ran warmer w/iridium than Tr55'*.
Cost difference was large between tr55'* and iridiums, all other things seem equal. Sonoma said he felt more power with the 55'*.
I doubt I'll go back to iridiums in my cars.
00 Regal GS: All kinds of cool stuff.