the build up is probably nothing to worry about. Like I like to say: Soot happens. (Cracks me up to say this out loud...)
Anyhow, there are 2 converters between the cylinders of each bank and the exhaust tips. Anything at the tips other than oil or coolant dripping from one or both is more a reflection of how the converters are doing rather than on what's gong on in the chambers.
Just a shot in the dark: Have you changed your driving style? Have you switched brands of gasoline? Do you run the oil level too high, overfilled (above the MAX line)?
All or any combination of one of the above can affect to some extent how the exhaust tips look. But even after sometimes 2 to 3 hundred miles of non-stop driving when I pull one of my car's into a gas station to fill up the gas tank the exhaust tips have some soot build up. Well, not build up per se just a good coating. The coating is thinner than it looks. Once in a great while when no one is looking I'll run a finger tip through the stuff and its just a thin layer of soot. About the only time I've read the tips change their appearance is from track driving. Then the owners worry about that...
Still, pay attention to the engine how it runs.It could be the early warning signs of an AOS. Real early. Or it could be normal. Regardless, like Pedro points out the oil filler tube cap "test" is not always going to have the cap stuck down tight on the tube even if the AOS proves to be bad.
But your car has the pre-conditions for a bad AOS. It has one. You are using the car. Oh, you want me to be more specific? Ok, your car has 68,972 miles. That's the miles at which they have been known to fail. Also at 68,973 miles, 68,974 miles, and well, you get the idea. The AOS can go bad at anytime.
Soot happens.
Soot happens.
Hahahaha.
I needed a laugh.