Spark plug got oil on the thread...

[gazza]

Wonder if anyone could shed some light on this...

I decided to check my spark plugs as I'm about to change the oil and filters etc only the left hand side front spark plug has got oil on the thread (I'm guessing it would be all over the tip but has been burt off?)
There's no oil in the spark plug tube or visible leaks.
The tip of the plug looked all coated like rich mixture so I check the right hand side spark plug, but it was fine looked normal.
So the oil seems to look like it's just in the thread of that spark plug hole.

I know the spark plug was clean when I put it in there last year.
I'll put new plugs in and probably spray some carb cleaner down there and carry on like it never happened but just wondered if anyone had any ideas?

Thanks.