You will see from this slide that what creates the need to change oil is the gradual buildup
of carbon that is being held in suspension. You can also see from the contamination curves that although we change oil there is always a number of fine particles left in the circuit which
the new oil immediately picks up and puts back into circulation. Over a period of time it is
these particles that give us the buildup of dirt we see in an engine when carrying out repair
work.