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No.Viscosity is the ratio of the rate of momentum flux to the velocity gradient.Since the velocity is constant it doesn't have a gradient and the ratio is meaningless.The impedance of free space is a few hundred ohms https://en.wikipedia.org/wiki/Impedance_of_free_space
Ok, but when a word is in quotation, ''viscosity'' , it does mean exact strict definition.
Based on this the viscosity of light would be zero at infinite wavelength where space-time is fully expanded. In black hole, the opposite is true with the viscosity of the light reaches a maximum.
the ''viscosity'' of light
Viscosity happens because matter interacts with matter. But light in a vacuum does not interact with other light (unless you get to extremely high energies). So as I understand it, light does not experience viscosity.
I suppose the closest you could get to a viscosity would be when light interacts with matter such as glass and slows down. But even then there there is no interaction between light and other light (except in some nonlinear optical effects) and certainly never in a vacuum (with the exception you mention).