Viscosity and density of fluids.

zaveri

When the temperature of a liquid increases, its density decreases, so does its viscosity.

Liquids like silicones have a constant viscosity with temperature change. But the density varies.

The mechanisms are different. Density changes are due to the temperature coefficient of volume. Viscosity is a lot more complex and follows an exponential relationship with temperature.
If you have the time, please wade through these two articles:
1. <a href="https://en.wikipedia.org/wiki/Temperature_dependence_of_liquid_viscosity" target="_blank" rel="nofollow noopener noreferrer">Temperature Dependence Of Liquid Viscosity</a>
2. <a href="https://www.engineeringtoolbox.com/fluid-density-temperature-pressure-d_309.html" target="_blank" rel="nofollow noopener noreferrer">Liquids - Densities vs. Pressure and Temperature Change</a>