You would not beleive the intricasies of this subject. For instance, the "10W" of a 10W 50 has a different viscostity to the "10W" of a 10W 60.

In practice, with modern oils it is difficult to really get it wrong.

My (educated ) opinion:

Lots of cold start winter driving - 0W 40.

General all purpose - 5W 40/10W 50

Hard use, not too many cold starts, especially for engines with large clearances, 10W 60 (millers CFS is the best i have found)

Rick.