The sticky sludge that GP refers to is for the most part an emulsion of water, produced by combustion and/or from ambient humidity, and oil. If an engine is no

The sticky sludge that GP refers to is for the most part an emulsion of water, produced by combustion and/or from ambient humidity, and oil. If an engine is not run frequently or is used only for short intervals the water isn't driven off by engine heat. Engines that aren't run much must be maintained based on elapsed time, not duration of use, due to factors such as this. The author's vehicle is a prime sludge candidate, although not as bad as people who go years w/o changes. Loss of viscosity is a wholly separate form of degradation which along with accumulated Ph changes is responsible for the "every N miles" recommendation.