The dispersancy of engine oils, which provides information on the cleanliness of the engine, deteriorates as a result of additive breakdown, oxidation and acidic reaction products from fuel combustion.
Alongside IR spectroscopy, the dot indicates through a uniform distribution of soot particles whether the oil is still able to suspend impurities so that they can be transported to the filter and filtered off. The intensity of the dark colouring indicates the soot content. A transparent outer ring indicates fuel. The oil droplet does not spread if anti-freeze agent glycol is present.
The camera never lies, and in this case can convert the subjective perspective from a personal visual inspection into an objective and reproducible statistical assessment.