The weights of the oil refer to it's viscocity level - which in plain english means how thick it gets(how slow it pours). Multi-weight oil changes how thick it is depending on temperature. I don't remember off hand the temperatures and such but the basic rule of thumb is the higher the number the thicker the oil.

From webpage http://www.vtr.org/maintain/oil-overview.html :
"An oil with too low a viscosity can shear and loose film strength at high temperatures. An oil with too high a viscosity may not pump to the proper parts at low temperatures and the film may tear at high rpm."

You should choose a lower viscocity range oil in the winter when the temperature is low. This allows the oil to flow quicker - I use 5w-30.

In Summer, I choose a 10W-40 (thicker oil)

2004 6mt Laser/Graphite/Aero/Premium/BSM/splash