Cobaltchat.com

Volvo recommends their own part number 3858996 gapped to .060 inches, stating you can go 300 hours between changes instead of 100 hours only if you use their plugs. But, it looks like they are the same as:

Champion RS12PYP (people report getting these in the Volvo boxes)
AC 41-932
AC 41-933 (platinum, newer)
AC 41-101 (iridium)
AC 41-993 (iridium) - many prefer these
NGK TR5IX (iridium) pre-gapped .040
NGK TR55IX (iridium) pre-gapped .060
Volvo 21467472 (4-pack?)

Note iridium plugs need to be gapped with care or the tip can crack - check out https://www.youtube.com/watch?v=lk70oyUEftY

Volvo says change every two years using their plugs, or every year if you use someone else's. But considering the Volvo comes with Champion plugs, it doesn't seem to make much difference in real life. I've read that people have had good luck with the AC 41-993 plugs too, and they're far cheaper than buying plugs from Volvo.

I myself if i I have a GM block I'm using AC Delco plugs. If its a Ford block I'll use Motorcraft plugs. I know in some Gm engines they use ngk if that's the case that's what I would use also. I'm not a champion plug fan at all unless it's in my lawnmower. Just me.