If the manufacturer's recco is known (it's on your radiator support, door jamb, owner's manual, and every Clymer's and Chilton's ever written) use it. Unless, of course, you have an advanced engineering degree.
Otherwise, select the 1st number (before the "W", which is "winter" not weight) on the basis of the coldest air temperature the engine will see before the next change. Too high: very slow cranking + oil doesn't reach the farthest bearings for 30 seconds. Too low: no oil pressure at idle.
Select the 2nd number on the basis of how hard will be engine be used + how loose are the bearings.
Ecuador: 20W, South Pole: 0W.
New engine: (W)30, race or ancient: (W)50.
How about straight 40 weight? Great for Venus, otherwise no.
