Adopted, NOT adapted.
It turns out that the Carbon build is on top of the valve an area where engine oil does not enter. So engine oil change interval had no effect on the Carbon build up. Pls correct me if I missed something! M/c used some instrument to inspect via spark plug hole - I am guessing.
The PCV system allows crankcase gasses which contain oil vapor to be reintroduced into the engine to be burned which comes in contact with the backside of the intake valves. This, along with the EGR gasses causes deposits to form on the valves.
Tester
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M/C used a camera and light on a thin flexible probe (Docs call this an endoscope) into the intake to look at your valves. It is the intakes that usually have the problem.
Thank you @Tester
So misfire could contribute to this?
https://www.howacarworks.com/basics/the-engine-how-power-is-created
After cleaning-out the PCV line and the PCV valveâand using Seafoamâyou should make sure that you use only Top Tier gas in the future.
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Oil can get there if the valve stem seals are leaking.
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The valves in your cars will benefit from Top Tier gas, they arenât direct injection. A bottle of Techron would also help with any valve deposits.
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Just in case the OP doesnât know which brands are Top-Tier certified, here is a link:
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I used Marvel Mystery Oil in my gasoline after my 1984 Chevy 2.0 engine started some preignition knock when accelerating. I thought carbon buildup may have been the cause. The problem went away and I kept using the MMO. Somewhere under 200,000 miles I had the head off and the machine shop guy remarked on how clean the innards were compared to many he saw.
Thanks @texases - I have been using Costco gas for like 10yrs.
I also like this additive https://www.amazon.com/Red-Line-Complete-System-Cleaner/dp/B000CPI5Z0/ref=sr_1_4
Seform is in my mind nowadays - might give it a try one day via manifold
Update:
I do not know if the engine oil change interval is not good enough that PCV gets damaged and results in Carbon build up on valves?
Then you will know from exhaust smoke and needing to top up engine oil - if I am not mistaken.
How often would you change engine oil, miles/months?
On my car the oil filter is on the back of the engine - I used to do it myself and then started taking it to shop. With Full Syn, I was changing less freq - now this Carbon issue.
Any symptoms of carbon buildup? How was it detected and where in the engine?
My oil changes: probably close to whatever was in the owners manual. Every 6 months or 4-6,000 miles was common in the days of my 1984 Cavalier.
???
The color gets dark because the oil gets dirty. Did this other guy think someone is putting dye packets in the filter?
If you have a direct injection engine, just resign yourself to getting/performing an induction cleaning every once in awhile. Unlike older designs, fuel no longer washes over the valves in DI engines, which means whatever lands on the valve stays there instead of being flushed away.
And yes, seafoam is fine to use in the intake, and in the gas tank, and in the oil - though if you use it in the oil you should do it shortly before an oil change, because you donât want it staying in the oil too long.
So he was saying that the oil filter was not good enough to filter the dirt that oil color became dark. But even if the filter is good enough to filter the dirt away, as the oil go through the filter, it will inherit the dirt color right?
@shanonia - the M/C used something like an Endoscope to inspect the valves via Spark plug hole. He said there is above average carbon on top of the valve.
âNot good enoughâ isnât really the proper term. No filter will filter out everything. After all, if it filters out everything, it filters out the oil as well, and itâs called a cap. 
It filters out particles down to a given size. Anything smaller will pass through the filter. If the smaller thing that passes through the filter is not clear, then the oil will darken. It doesnât mean the filter isnât âgood.â
As for inheriting the dirt color, the oil already has the color from the particles suspended in it that donât get filtered out. Anything it picks up by flowing past larger particles which are trapped in the filter would be indistinguishable from the stuff itâs already carrying.
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Sounds like he tried to sell you an engine decarb treatment and he failed. Most people donât care about the carbon in an old engine.
Good hunch. I donât think OP has any carbon buildup problems at all. He hasnât reported any symptoms. If the mechanic has diagnosed carbon as the source of a problem, we havenât been told.
If it were my car and I wanted to do something on my own, I would try MMO in the gas for quite a few tanks full. Itâs supposed to soften the carbon deposits, which get expelled through the exhaust valves. Thatâs less dramatic than some âtreatmentsâ and possibly more thorough. But with no problem, it is not a solution â more like TLC that may or may not make a difference,
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What is MMO?