0W-40 is the viscosity rating. OW is for extreme cold temperatures (sub-zero) and 40 is for very high temperatures (perhaps over 115 degrees F)

A range of 5W-30 here in Southern California is quite common, in fact the most common.

As far as synthetic oil goes you can read more about Mobil 1 at:
http://www.mobiloil.com/USA-English/...l_1_0W-40.aspx