Volvo brand spark plugs for '91 740

I've long used plain NGK plugs, ran Bosch Platinums once when they first came out and was not impressed, worked fine for a while but then started idling poorly, found that the center electrode had worn way up inside the ceramic.

Although I'm sure some will disagree, I've always felt it was best to buy 'em cheap and change 'em often, rather than buying expensive plugs and hoping they will last 50K miles or so... I prefer Champion, they seem to thread in easier than AC plugs, but that's just a feeling, no science. I clean them and check the gap around 5 or 6K miles, change them around 10-15K miles, depending on the weather! (no sense working in the cold if you don't have to!)...