16k to 20k mile changes? I'm sorry but that is just stupid. That is why you have sludge in your engine. I don't care if you use Mobil One or Amsoil, that is just to long. I can't understand why people try to stretch oil changes it is the most important fluid and also the cheapist and simplest to do. We do not drive comercial equipment, our engine's do not hold many gallons of oil. If we drove 100k+ per year and had engines that held 5-10 gallons of oil with two expensive oil filters. Then yes trying to stretch changes can make business sense but not for private cars. My advice is to sell it and learn, it is not worth your time and aggravation.