Viscosity is important because it allows the whitening gel to stick to your teeth. This is important to the whitening process.   Medium and High Viscosity gels are used for dental bleaching.   If a gel is not viscous enough, it will leak out of the bottom of your bleaching tray, which slows the whitening process and creates unneeded sensitivities. As well, saliva can dilute a low-viscosity gel, rendering it non-effective.