Hi all, I replaced the plugs last week and found one of them has oil/grease just like the picture below. The mileage was about 80k. Any idea of what it is exactly and why it's there?
Thanks!

 

Looks like just discoloration of the insulator material to me--I've seen older plugs do this. Was there actual oil there, or just dry discoloration? The most important thing is what the electrode surfaces look like--should be light gray-brown.

You can also get some "blow by" if the plug was not installed tight enough--usually a black powdery soot.

The only other thing might be a leak in the valve cover coming down into the plug compartment. Good luck.

 

That brown ring is normal and would normally be on all 6:
The brown ring is normal; it’s nothing more than an electrical charge that is between the porcelain and metal. Dirt and oil mist is being attracted towards the spark plug by the electro-static effect of the high voltages. No problem there.
Now if it were black, I'd venture a guess that the plug was loose and it was a carbon ring.

 

^^^Yup, perfectly normal. Known as "corona". What would seem to be abnormal is if it's the only plug with a corona....

And what plugs did you use as replacements? If the other plug in the picture is what you used, then you bought the wrong plugs....

 

^^^^yep. Use the exact OEM spec NGK replacement plug.

 

Definitely Normal. Oem plug should be an NGK IZFR6K-11 . you could probably pick these up at an auto parts store, just make sure the number matches up . At AutoZone, their part # is 6994 NGK Laser Iridium and the plug in the box should have ^ that number on it.