Crude oil is not a pure substance - ie. crude oil does not have a specific definition, like say O2.
Crude oil is a mixture of a whole bunch of HC, water, and other stuff.
The fact that 2 crude samples have the same density only means that they have the same density. The chemical properties of the 2 samples will most likely vary widely - like viscosity. Viscosity variability has more to do with composition than density/temperature.
In school, we had correlation charts for specific crudes. Even this is not reliable, a field usually has many wells, and also within a "well" it may vary from location to location, within the same formation.
In practice, any viscosity vs density/temperature will only be of use for the particular crude you are looking at.
In my industry, we take it to the lab and they measure it.