I see some people posting that conventional oil can't keep an engine as "clean" as a synthetic over time. I am a 210k mile user of dino (4k and Valvoline mostly) except one Mobil 1 run. The car still achieves 35 mpg average mileage and does not seem to have lost much if any power. I saw the valve cover off and just saw a mainly gold varnish on the engine components underneath. I have never put one additive or cleaner into the car. Anyway my question, I imagine my motor is not "clean" but what will the "clean" motor get me? Lastly what are the determental effects of a not "clean" motor etc.