In lubrication terms, which term describes the increase in lube oil viscosity due to aging or contamination?

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When a lubricant ages or gets contaminated, its flow becomes more resistant because the oil’s molecules and additives undergo changes that increase its resistance to flow. This overall rise in viscosity is described as thickening. Aging can lead to oxidation and the formation of heavier, higher molecular weight species, sometimes even networks or deposits that hinder flow. Dilution would make the oil thinner, not thicker. Gelation is a specific outcome where the liquid becomes gel-like, and polymerization is a particular mechanism that can contribute to thickening by linking molecules into larger ones. The general term that best captures the observed increase in viscosity due to aging or contamination is thickening.