Which AC Delco plugs for a '95 3100?

[atikovi]

Conventional, Rapidfire platinum or OE Double Platinum? Are the OE worth double the price of the Rapidfire plugs?

[Schurkey]

"Conventional" plug will work.  For awhile.  They wear quicker than the Platinum or Iridium plugs, and they require more voltage to fire.  If you don't have problems with oil burning or leaking injectors that would tend to foul the plugs, you'd do well to buy longer-lasting plugs to begin with.

"Platinum" (single-Platinum) plugs only work properly in vehicles that have a distributor, or that have coil-per-cylinder ignition.  On vehicles with paired cylinders and waste-spark ignition, (three "coil packs" and six cylinders, for example) half the spark plugs fire in the normal polarity, the other half fire in reverse polarity.  For the cylinders firing in reverse polarity, the single-Platinum speck welded to the plug is on the wrong electrode to do any good.

Thus the need for Dual Platinum (or dual Iridium) plugs on a waste-spark ignition system--no matter which cylinder they get installed in, there's a Platinum (or Iridium) speck on the correct electrode.  You get all the benefits of a fine-wire center electrode including low firing voltage, plus the benefits of the precious-metal lump including long service life (if the plug doesn't foul or otherwise get contaminated.)

Edited February 9, 2022 by Schurkey