how does cooling effects viscosity?

Answer 1

Sometimes it is a matter of what substance you're talking about.

In the case of lava, cooling increases viscosity because it is becomming more solid and forming into rock,- while heating decreases viscosity. This is because particles move faster when they are hot, and as a substance cools down the particles slow down, creating more viscosity.

Answer 2

Usually increases viscosity (e.g. things tend to get thicker, more viscous, as they get colder)