I pulled one plug on my 73,000 mile car and it shows just a little wear. Are these ment to go 100K or just 60K? The car runs fine and the plug looks as if it will make it to 100K so can I just leave it be? If not, what is a good replacement. Boschusa.com lists the factory replacement as F8DPP332 and AutoZone sells them for $9 each (and the car takes 16 plugs) or over $150 with tax. But oddly enough, they also list their Platinum2 and Platinum+4 at $4 and $6! Why is the regular platinum plug so much more than the supposedly better P2 and P4 plugs?