The ascending numerical filename order is 1-3-5-7-2-4-6-8,
front to back, left to right. 

7 is the bad one. I've never seen deposits look
like that. The wet appearance stems from me thinking
I'd plunged the tip of the plug into oily schmutz 
fishing it out of the engine bay, so I hit it with
spray carb cleaner before looking closely. 

After chipping off the bulk of the deposit the plug
would spark on a lawnmower. A casual crank on a compression
gauge showed 90 psi with closed throttle, so I reinstalled
it and ran the engine a bit. The idle is faster (~800 RPM)
but still quite rough. The AIR pipe leading to the cylinder
warms up about as much as the others, so I think the plug
is firing the cylinder but the plug isn't the whole story.