something tells me that those were the wrong plugs for the job. I think that the brown marks around the base could be due to overheating. So this says that you have been driving long distances on hot plugs, that is plugs that don't dissipate heat quickly enough for long drives, they save a bit of gas if you only drive short distances cos they heat up quicker. With the new NGK plugs are you finding that the engine runs a bit cooler? I dunno, I could be wrong
well i havn't gone far enough to tell yet, the temp gauge is in the middle like usual.
i did wonder about that at one point.
just today i realized that these may be cooler then the bosch, but they arn't the cool type. i should have asked for a BKR6E instead. its probably too late to bring the plugs back, i already drove about 15 miles on them.
The real reason that you see oil around the base is that the oil seals around the valve cover are hard and are leaking into the plug holes. You need new seals on the cover on all plug holes. They are tough to pul out of the cover, so be patient. I had the same issue, I thought I was burning oil, but if the insulator is a nice whiteish/slight grey color wihtout pits on the ceramic insulator, you are right on with the burning.