The lower the "W" number the better the oil's cold temperature / cold start performance.
Two series of viscosity grades :
(A) those containing the letter W
(B) those without W.
(A) Single viscosity grade oils with the letter W are defined by maximum low temperature cranking and pumping viscosities and a minimum kinematic viscosity at 100C.
(B) Single grade oils without the letter W are based on a set of minimum and maximum kinematic viscosities at 100C and a minimum high shear rate viscosity at 150C.