The oceans are becoming less viscous as they are heated. I'd imagine a similar effect is likely occurring in the atmosphere as well. What, if any, effect would this reduction of viscosity have on aeroplanes? Would they be more efficient and use less fuel?
As far as I remember viscosity of the gases, unlike liquids, increases with increasing temperature. So in order to decrease viscosity, You would have to cool the air (which is already cold at some 10 km).
When it comes to it's effect on planes, viscosity is responsible for creating pressure gradient between top and bottom side of an airfoil or wing. That pressure gradient creates lift. On the other hand viscosity affects drag - generally the more viscous fluid, the greater drag.
There is no easy answer to Your question, but if You would like to improve flight conditions, I think it would be easier to start with the plane construction and materials, instead of the air viscosity.