I use 5W-30.
5 = the viscosity when it's cold. I forget exactly what temp that is, but I think it's 0*C. If you drive in cold weather (,50*), don't go higher than 10. You need to get the cold
oil up into the cold engine as quickly as possible.
Cold thick
oil can also damage the
oil cooler if you blip the throttle.
W = "winter", not weight.
30 = it will have the viscosity of 30 that has been heated to 100*C. That doesn't mean it has a viscosity of 30 when hot.
You need enough viscosity to give the proper oil pressure, whatever you decide it should be. Thicker oil wastes energy, adds wear to drive parts, and provides no additional benifit.
Keep in mind that the oil pump has a bypass spring. If you're getting 50psi with 5W-30, you won't get much more than that with 20W-50.