Liquid viscosity resists surface formation. If the viscosity is great enough, a nozzle may produce a mass of filaments instead of a spray. In the photos shown below, the same nozzle is used to compare two different fluids and viscosities being sprayed. The nozzle to the left is spraying water and on the right oil with an approximate viscosity of 111 cP. Fluids with a high viscosity, like oil, can reduce the spray angle and flow rate, which can result in poor spray coverage. This is why selecting the right nozzle can make all the difference in spray performance.
Temperature also affects viscosity because at higher temperatures the mass of the fluid can be reduced or thinned. Take for example, honey, which is fairly viscous. It will flow, slowly, in response to the force of gravity. When removed from the refrigerator, it will flow very slowly, and with a few seconds in the microwave, it will flow much more easily, and swiftly. Therefore, viscosity of a particular fluid changes as temperature is increased or decreased.