To answer your last question first, yes that is exactly what Rule .308 is. Way back when I had to come up with a username I had more .308's than .303. I'd say that it is pretty much a coin toss as to which one I have more of now.

On the spark plug question, for the most part I detest both of those brands equally but if they were my only two choices I would hold my nose and install the Champions and order a set of factory Volvo plugs just as fast as you can. The turbo plugs are platinum from the factory.

There really are not a lot of tell tale signs that someone has been in there changing out the plugs. You say it'll be interesting to find Volvo plugs in there, did you pay to have some kind of wildcat plug put in there before? I think that it would be even more interesting to get in there and find the wildcats that you paid to have installed and to find out that they are the root source of your problem.

When you do screw new plugs in it do yourself a favor a put a little dob of anti-seize on the plugs. Whomever the next guy is that changes them out will thank you. It is a steel plug going into an aluminum head, they have been know to galled together and cause all kinds of nasty problems, a little anti-seize goes a long ways in this application so don't over do it.

Mark