choose weight based on bearing clearances and oil temperature.
W weights are specified based on 'cold' temperatures, the other weights are specified on 'hot' temperatures. "hot" and "cold" are in quotes because they are SAE specifications, not what we normally call hot or cold when deciding to put on a jacket.
So if you're engine's oil temperatures run hot, like 280 F and it was built with fairly standard clearances, a 50 wt might be appropriate. But if runs cold, say like 190 F, then pressure in the pump will be high, but the relief valve will be open and much of the oil will get dumped back into the sump. At high rpm, that can be bad. For oil to support all the bearings and get to the top there needs to be enough volume as well as pressure.
Good information about SAE wts vs. viscosity and pour points on Richard Widman's Website.
In particular download his white paper on "Choosing engine oil for Corvairs", which also delves into other physical characteristics, explains the five basic types of motor oil, and a bit on additive packages.
Pdf in English from his hobby pages http://www.widman.biz/Corvair/English/Links/Links.html
or his business pages http://www.widman.biz/English/Selection/oil.html
A quick slick approximation of grading (weight) to viscosity are the tables on this page http://www.bobistheoilguy.com/viscosity-charts/
For more on the issue of blowing the oil out the relief valve, see "Haas on Oil" aka Oil 101.
http://ferrarichat.com/forum/faq.php?faq=haas_articles
Note when Haas mentions "track" he means road course - where an engine gets really heat soaked, not 1/4 mile drag strip. Also, if you reference the visocity tables for oil grades, you'll see where his "thickness" numbers come from.
