Synth oil in cold weather

["Mike " <JoMi@msn.com>]

Just changed the oil in the 91 200q. Used the same Mobile 1 15w 50 as I always 
do. Curious as to why people change to a different weight oil in the winter if 
using synthetic? Mobile 1 is supposedly good to -60F(or so) so why change to 
10w 40 or 10w 30 synth in the cold weather? Am I missing something? What is 
the advantage of a lower viscosity synthetic in cold weather?

mike miller - seattle, wa
where 20F above is a cold day
but may see 20F below in Montana
later in the winter with 15w 50.