How well does "serum viscosity" blood test correlate to "whole blood viscosity" ? under what circumstances could 1st be norma and 2nd be high ?

Red blood cells: Red blood cells and fibrinogen are present in blood but not in serum. Polycythemia, sickling, hyperfibrinogenemia might be your explanation.

Blood viscoisty: I do not think any such measure like whole body viscosity. Serum viscosity reflects the thickness of the blood which is inside the blood vessel.

Inflexable RBC's: Syndromes of hyperviscosity, due to increased whole blood viscosity may be caused by reduced deformability of red blood cells, often evident in sickle cell anemia, and polycythemia vera.