Castrol synthetic or Valvoline?

Hi,
Most manufactures seem to recommend 5-30 for engines of all sizes now days. When I bought my 99 Dakota with a 5.2, I did some net research and it seemed 5-30 was most widely used for magnums, likely due to manufacture recommendation. However, on the net there were many hesitant to run that, and they ran 10-30 instead. My perception from what I read is that they didn't seem to have real data reasons for not running 5-30. It seemed that their hesitance was based on more of a old school perception thing.

Travis