What is detergency and dispersancy?
Detergency refers to the oils ability to keep engine components clean, particularly those in the hotter parts of the engine (pistons, rings, valves, etc.). The additive is referred to as a detergent.  Dispersancy refers to the oils ability to keep solid contaminants (i.e. soot, combustion debris, etc.) in suspension.  This is necessary for two main reasons:  firstly, it delivers the contaminants to the filter where the bigger particles are removed and secondly, it ensures that all the smaller particles flow out when the oil is drained, leaving the engine clean.
