1. For use in logging downhole in an earth borehole, a method for determining a characteristic of oil that may be present in a downhole measurement region, comprising the steps of: providing a plurality of crude oil samples; determining the viscosities of the plurality of crude oil samples; performing nuclear magnetic resonance test measurements, using a predetermined operating mode having operating parameters impacting relaxation time and hydrogen index measures of the plurality of crude oil samples, on media including said plurality of crude oil samples to obtain test data; applying a test processing procedure to said test data to obtain an output test parameter relating to said test data and said predetermined operating mode; deriving, for said plurality of crude oil samples, a correlation relating said output test parameter to the viscosities of said crude oil samples; performing downhole measurements in the measurement region with a nuclear magnetic resonance logging device, operated with substantially said predetermined operating mode, to obtain logging data, and applying a measurement processing procedure to said logging data to obtain an output logging parameter; and determining the oil characteristic using said output logging parameter and said correlation.