Originally Posted by StephenVA

Black and dry is a sure sign of poor combustion. Wet and dark is an oil control related issue. Since I assume you do not have access to a lab scope, I would recommend replacing the coil on that cyl or at the least swap it for another location to see if the "issue" moves with the coil.
Issues Questions - Cyl misfires under load? Hard starting on hot soak? Idle stable or "hunting"?
Ignition Issues - Coil or plug (Or both) Don't forget about connection failures
Fuel delivery - Injector leaking, stays open (drips or incomplete spray pattern), clean system with injector cleaner. Better? Y/N, No move to injector removal and ultrasonic cleaning and new seals.

Quick tests
With a digital volt meter read the resistance of each plug (one higher than the others?)
Connect a scanner to the OBDll port, display live readings. Before starting check the stored codes and see if there is any indication of engine cyl issues.
Disconnect the problem cyl coil for 15 secs and record RPM drop, then do the others on the same bank. 10% greater difference from any location? Note O2 Sensor readings during tests. Any wide variations between cyls when coil is killed?
You can alway do the same test with each injector.
Clear codes when complete as the module will record cyl misfires.

The single file wire plugs will provide a better ignition burn than any given multiple prong plug in our applications 6/8. Those reporting otherwise have ign issues, fuel issues, or oil control problems, all of which can be typical symptoms of aging systems. Time to clean combustion chambers, injectors, replace coils, etc. Lab scope, 4 gas meter, and proper diagnostics will prove out everytime.

Helpful?