I use Hicks brand Collector Choice Oil (or Muscle Car Oil, same stuff) 20-50. Has the same amount of ZDDP as the Valvoline VR1 racing oil (which from my research has the highest content) but it's cheaper. NAPA stocks it but I'd call first to make sure they have the viscosity you're looking for on hand.

http://www.hicksoils.com/pdfs/10-13-09/Collector%20Car%20Motor%20Oil%20PDS.pdf

http://www.hicksoils.com/technical.asp

They also advertise private label packaging and I've heard it's what Hemmings is selling under their own name.

1951 Chevy 3100
In the Stovebolt Gallery
www.rustyfiftyone.com