2004 Sienna Carbon build up inside the engine??

Hello everyone,



I change the oil in my 2004 Sienna every 3,000 miles religiously. I just changed the oil two days ago and noticed that between the last oil change in Oct 2010 and now, there was a carbon build up in the opening where I put the oil.



Has anyone had that problem?

Can you tell me what could be causing this issue?



Look forward to your comments.