Hey Jim:
Keep in mind that heavier viscosities (20W-50) give poor start up lubrication, reduce power and do not easily get through the extra hoses and
oil cooler most Cobras have.
I run 5W-30 Mobil1 and my pressure never goes below 50PSI (351 Crate). I would not be custom blending straight weight oils, 2 quarts of 10 weight, plus 2 quarts of 20, plus 2 quarts of 30 does not produce 10W-30! Do not use the Fram filters, use Wix or Mobil1.
Bob