Carbon build up on Valves? Prevention? Treatment? How often to change oil?

Current state:
M/c fixed the vacuum leak. Lean condition became Rich condition that he is investigating now. I tried to suggest that the computer needs to re-learn! He also did not test fuel pressure.
Fuel Trim - Originally:
1500 rpm 2500 rpm
LTFT 6.2 6.2
STFT 6.2 8.6

After I disconnected the battery and let all the Capacitors discharge:
LTFT - 4.6
STFT - 7.8 - 12.5

Originally:

  • idle random misfire P0300, P0304 and occasionally P0303
  • Changed spark/ front O2 sensor (OEM) - Spark is platinum a level up from OEM
  • Changed cap/rotor, spark wires with aftermarket parts - still misfire
  • Swapped the Dizzy coil but still misfires
  • Engine compartment is dry except Dizzy neeeded a new O ring - took care
  • Compression was good 175 to 190 (renage 135 to 199)
  • M/c did the leak down test and no issues
  • I added S1 fuel injector cleaner and the car started running fine (for a day!) but then misfiring closer to 2400rpm on road and not on P gear. Misfires on all Cylinders. Basically it misfires at idle and stops once warmed up. But on the road, it misfired around 2400rpm.
  • Owned the car for over 15yrs with little issues.Last yr IACV had to be cleaned. Carbon build up stopped its functions temp - after cleaning it started running like a new car. Throttle body is clean.
  • Use Mobil 1 5W30 every 5k.
  • valves adjusted 22k miles ago
  • air filter has 8k
  • Spark plugs did not show any abnormality
  • Batt and Alternator tested fine
  • cold start gives gas smell through exhaust even before the misfire
  • only known issue is the car would need jump start if you don’t drive for 2 weeks

Related thread:
https://community.cartalk.com/t/are-the-cap-rotor-still-good/175532

Update:
I did edit here and there and added new info.