wuli,
Viscosity index is calculted between 40°C and 100°C.
"To access this property of a lubricating oil the American Society for Testing and Materials (ASTM) created a method to provide a number called the Viscosity Index (VI) which correlates the amount of viscosity change for a given oil compared to two reference oils having the highest and lowest viscosity indices at the time when the VI scale was first introduced (1929). A standard paraffinic oil was given a VI of 100 and a standard naphthenic oil a VI of 0. Tables have been prepared which show the relationship between viscosities at 40 and 100°C. The method has been updated and revised several times to include VI values higher than 100.
"The important thing to remember is: a low VI means a relatively large viscosity change with temperature and a high VI denotes a smaller change of viscosity with temperature. Hence, the VI of an oil is of importance in applications where an appreciable change in temperature of the lubricating oil could affect the start-up or operating characteristics of the equipment."
http://www.fammllc.com/htmlpub/LUBE_bulletin_06.htm
Also see "viscosity index" @
http://www.chevron.com/prodserv/NewOronite/library/li_dictionary_v.htm
A 15W- oil of any composition must have a viscosity no more than 7000 centiPoise @ -20°C. A 5W- oil of any composition must have a viscosity no more than 6600 cP @ -30°C.
So...by definition, if the oils are correctly labeled, the 5W- oil is thinner when cold than the 15W- oil.
As your link says, "...PAOs require fewer VI improvers [to achieve the desired viscosity index] and are less likely to create performance-limiting deposits."
Ken