In candle making viscosity pertains to the thickness/heaviness of wax. An example is the difference between pure paraffin wax vs. beeswax. The same wick will not work in the same size candle jar. Paraffin burns almost twice as fast as beeswax. It's thinner & lighter so it carries up the wick smoothly. It takes a wick generally 3 sizes larger to burn the same candle made with beeswax. Kind of like the difference between olive oil & motor oil. I hope this helps.
Regards,
~ pat ~