Many people refers to viscosity increase as oil thickening and viscosity decrease as oil thinning. What do they mean by thickening and thinning? Is it the oil film?

Viscosity is an oils resistance to flow and shear. Kinematic viscosity (measured in centisokes or cSt) is measured by timing how long a fluid takes to travel through a point of resistance. A thinner oil would pass through quicker then a thicker oil. The thickness of an oil is dependant on the length of the carbon chain. The longer the chain the thicker the oil. Viscosity of oil can be affected by moisture, temperature, contaminants and chemical changes.

Duarte,

It is hard to add anything more after MC laid down the facts so well. But, to answer you question; yes, at a certain temperature, oil film is directly affected by the oil’s viscosity. So one can say that “thinner” oils will form thinner oil films, and “thicker” oils would provide thicker oil films.

Duarte
It may be too late, however, I will add: Thickening is a controled process in order to gain extra capacity to support loads and high temperatures. and thinning is a control process in order to get fluidity. Both to get the right oil film.

For general industrial use the above answers apply however, if you are dealing with diesel engines oil thickening and thinning are monitored and trended as a measure of soot load (thickening) and fuel dilution (thinning).

My question was more addressed to the physical (or scientific) meaning of oil thickening and oil thinning in terms of viscosity increase and viscosity decrease, respectively. Thanks for the answers.