I think 20W-50 would be a very bad choice for a freshly rebuilt engine, even if you were living in Central America. Yes, the higher viscosity provides more of a "cushion" which in a freshly rebuilt engine you really don't need, but it comes at a trade-off in flow which means slower times for fresh oil to reach critical areas, and consequent heat removal. Use a good quality brand multi-vis and select a type whose higher viscosity# matches what your Packard owner's manual suggested for temperatures you expect to encounter. And a freshly rebuilt engine should especially have an oil on the light side and be changed after perhaps 100 miles, and again after perhaps 500.
There was a really excellent article recently referenced on this forum about choices in motor oil which I recommend highly, link below. The writer is a Corvair owner so just read around that part of it. Especially read the section towards the end as I recall, about how important viscosity selection is, and also about engine damage that can result from excess ZDDP and other additives.
http://www.widman.biz/uploads/Corvair_oil.pdfThis Post was from: https://packardinfo.com/xoops/html/modules/newbb/viewtopic.php?post_id=150505