I know the manual(s) that came with your car probably say 0.023" (0.5mm?) spark plug gap - but 0.032" (0.8mm?) is the proper value. Running 98 is a GOOD thing on the 3.5 - it has high compression, and lower octane tends to ping a lot on it. Since it requires a hotter spark to ignite higher octane fuels, I would suggest the W7DC (the hotter one I believe, but if it's the 9 that's hotter, use that). Hotter plugs can reduce pinging and incomplete combustion, as well as warm up to their self-cleaning temp faster for easier starting and better idling. The original suggestions for the 8 are taking 2 things into account: 1) longer plug life (colder spark in a hotter than avg chamber to lengthen life) and 2) Lower grade fuel doesnt need as hot a plug. I say since you're a DIYer, plug life probably isnt as important, plus modern plugs tend to last a lot longer than the originals.