This is the whole reason for multigrade oils.....
When cold, a multigrade oil acts like it is the lower of the two figures, ie the 20 in 20w50, and 50 when it is hot, the higher number being the thicker oil.
As the oil heats up the some of the polymer molecules (that are added to make an oil multigrade) expand making the oil act as if is was thicker when hotter, although as it is warmer, it appears thinner. the CHARACTERISTICS of the warm oil is of a 50 weight oil, where as the the cold oil is like a 20 weight.
So.... you get thin a characteristic oil in the morning when cold to allow easier starting, and a thicker oil when hot to maintain oil pressure.
It removes the need to run 30w oil in winter and then change to 50w oil in summer when things are a lot warmer.
You can read a bit more here: http://en.wikipedia.org/wiki/Engine_oil#Multi-grade
HTH,