My conclusions after doing a clean was that yes there is carbon buildup but that it wasn't enough to make any real world difference. Other cars it is very noticeable....a good friend of mine drives a little Hyundai accent with a DI engine and oh man the carbon is bad. Lots of oil too, was like mud when I cleaned it last. He noticed a significant drop in power and fuel economy. Just to quickly experiment (didn't have much time then) we just sprayed some throttle body cleaner right onto the valves and it made an immediate difference.Carbon buildup in the valves/intake isn't something that would be found in UOA right?
With the few fellas surpassing the 300k mile mark, for the majority of us with the 2.0/2.5L motors, I presume we are fine without ever never "needing" walnut cleanings.
It is a bit unfortunate that we don't really have a record of how the OP's vehicle was utilized in the first 60k miles of it's life, so is it safe to guess that it was mostly city driven? Probably right?
Our cars aren't like that. From what I found and from what we have seen on this forum, I would suspect that for virtually all of these cars out there it isn't a problem and not worth doing anything about until the car is old enough to be scrapped anyway.
For those that it is a problem it seems that how much they drive it is the issue. All seem to be driving short trips in city. The amount of oil the engine goes through is another factor. Mine doesn't really go through any while we have a few members that need to add oil between changes. Seems it's getting into the intake. For those, I would put a catch can in. For myself, there's no point.
In short, this is a problem for a very small number.