I would be very careful in calibrating Ubbelohde viscometers. They work as well as your calibration is. It is better to calibrate with a liquid whose viscosity is very near the viscosity of the liquid you want to measure. Water is definitely a good callibrant and so is toluene. For very high viscosity liquids, there are oild, I prefered squalane. Just be careful
I would second Marc Assael's comment. Good references are H. Bauer & G. Meerlander, Rheol. Acta, 1984, 232, 514 and W. Kunzel. H. F. van Wijk and K. N. Marsh, Chapter 4 "Viscosity" in K. N. Marsh ed., Recommended Reference Materials for the Realization of Physicochemical Properties, Blackwell, 1987. (IUPAC).