Varnish is a good generalisation for the conglomeration of sediment products that can build up after extended degradation of the base oil; it effectively describes that yellow/brown sludge that you may have noticed in tanks or on used filter elements.
What causes the formation of varnish?
Oil degradation describes the fracturing of oil molecules and the constituent additives; these broken molecules can ultimately stick to each other, as well as other contaminants present, to form varnish. But what is the cause of oil degradation in the first place? The process is called oxidation, which essentially describes the reaction of the base oil with oxygen.
Varnish (oil oxidation products) is a serious threat to machinery condition and needs to be monitored appropriately. MPC is a reasonable measure of varnish potential within a system but further verification of MPC testing is important; this can be achieved through Particle Counting at 80°C as well as 20°C. If a high MPC value accompanies a decrease in the Particle Count, we can confirm the presence of varnish.