I changed out the COPs in my 2006 F150 5.4L the other evening and the #6 cylinder COP smelled like burnt oil. It also had a slight brown color or the boot. There was no actual liquid on the boot or in the plug well. All the other COPs were black and had no unusual odor to them. The spark plugs have no more than 6k miles on them. I didnt pull the spark plug to look at it but if just kind of figured they are okay. Anyone have any suggestions? Thanks