To my surprise, there's no oil residue on the spark plugs or holes, so I guess it's just the faulty valve cover gasket. Plugs #2 and #6 seem to look okay for 45k miles, but #4 doesn't look so great. I took a peek at the piston tops, and it has a consistent dry rust look. All of them look the same.
About 2 years ago the Z was exposed to extreme summer heat with ambient temps averaging 110-114F for several days since it was still my daily driver. It nearly overheated several times while idling with the A/C, and there was burnt plastic smell as well. I quickly "limped" home on the freeway to keep the temps down and quickly parked the car. I later switched the HFCs to TPs and had the hood vented. The car hasn't overheated since.
Anyway, I'll try to get the driver side valve cover off this week as I wait for parts.