Sorry, been in and out here and dealing with a close friend driving a Ninja under a pickup. OEM plugs are Delco Double platinum clad. Supposedly good for 90+ miles. I use Autolite Copper because I get a nitrous hair up my butt often. It eats plat and irridium fast. I would stick with OEM from GM Parts. You'd be hard pressed to find another plug thats proven to go 100k time after time as shown in Caddys for a long while now.
I let the local Caddy Dealer do my plugs and aint afraid to admit it. I dont want to deal with it. Several owners have done it and say it'* moderately to very hard. You need to remove the EGR and strut tower brace, have the patience of a saint, and 7 long effeminate fingers
. I was talking to a Caddy owner up to the local garage a few weeks back, and he said it'* alot easier to do from the back with the car up on a lift. Not sure on that one. I'm to fat to fly, or get further under the front than to pull the oil drain plug in my gravel driveway.