I spoke to my mechanic. He said that the rings are fine and the oil on the plug threads are anti-seize compound (i feel really stupid). The carbon, he thinks, is from not driving the car long enough for it to reach normal operating temp (i only drive 8 mi a day - one way) OR it could also be the injectors. I'm using Bosch Platinum +4s but he doesn't see any problems with those. He said a couple of his customers claim to be getting better mileage with them.
I'll change back to normal copper core to be safe and start using injector cleaners. I feel much better. Thanks for all the advice.