Since, using displacers in high viscose liquids may result in problems, I would wonder about the limit of viscosity or in which services we other types of level transmitters shall be used instead of displacers.
I've always considered the use of buoyancy displacers suitable for clean fluids only because any build-up of material on the displacer changes its weight and therefore the 'calibration' of the unit.
If a material is viscous enough to adhere to, build-up, coat or encrust the displacer, then one risks the displacer falling out of 'calibration' as its weight changes.
I was curious what Magnetrol had to say about viscosity since they're a major player in this market.