What is the maximum temperature of viscosity?

Viscosity is a measure of how easily a given material will
flow. One way to look at it is how "syrupy" is the material.  A liquid such as water has
a low viscosity while molasses has a high viscosity.  Viscosity is related to the
temperature of the liquid.  For example, syrup just taken out of as refrigerator will
have a much high viscosity than the same syrup after it has been heated in a microwave 
before using on your favorite pancakes.  Therefore, when measuring viscosity, the
temperature at which the viscosity was measured needs to be
stated.

Viscosity is also used to describe motor oils used
in cars and trucks. Motor oils with a low viscosity will still flow even when the
ambient temperature is low whereas motor oils with a high viscosity will get very thick
when the temperature gets very low.

This change in
viscosity is why in very cold climates, like Alaska in the winter time, trucks keep
their engines running all the time. If they left them off overnight the oil would be so
thick and viscous that the engine would not start.

At the
other end of the temperature spectrum, as you increase the temperature for a given oil
you will reach a point where the liquid starts breaking down and no longer works as
intended. You could consider this the maximum temperature of viscosity for that material
but it would be different for every liquid.

The only way to
determine the maximum temperature for a particular substance would be to test it in a
lab and see when it starts breaking down.