Laboratory testing of a particular oil at various temperatures is the most reliable method of determining how an oil behaves. ASTM D 341 outlines a procedure where the viscosity is measured at two different temperatures and then either through a computation or special graph paper the viscosity at any other temperature can be obtained. Figure 6-4 shows a portion of one of these graphs.
As a rule, with crude of 30° API and higher the viscosity is so low that normally it may be difficult to find any information on file regarding a specific crude viscosity. Between 30° API and 11° API, the viscosity becomes more important, until in some cases it is impossible to process very low gravity crudes without a diluent to reduce the viscosity.
The use of a diluent is not unusual for crude oil below 14° API. With virtually any crude oil the viscosity change with temperatures can be an excellent guide to minimum crude processing temperatures. An ASTM chart of the viscosity versus temperature is useful to detect the paraffin formation or cloud point of the crude as shown in Figure 6-4. This normally establishes a minimum temperature for the treating process. There are examples of 30° API crude and higher that have pour points of 80° to 90°F. Crude oils of this type are common in the Uinta and Green River Basins of the United States as well as S.E. Asia. In the absence of any laboratory data, Fluid Properties discusses correlations that can be used to estimate crude viscosity given its gravity and temperature.