Oil viscosity was determined using a Bohlin C-VOR 150 viscometer
(Malvern Instruments Ltd, Great Malvern, UK), equipped with
a temperature control unit. A rotating bob (coaxial cylinder C25)
and a sample cup were used for this analysis. An oil aliquot was
poured into the cup before placing it inside the water-jacketed
compartment. When the temperature of the oil sample reached
40 C and remained constant, the rotating bob was shifted down
into the cup and the viscosity measurement was taken by fixing
the shear rate at 100 s1.