I checked Castrol's web site where they have different country products and it appears there is this classic oil available for example in Germany but not in the US
Castrol CLASSIC OIL XL SAE 20W-50
For pre-1980 modern classic cars
A conventional multigrade engine oil of good all round performance, formulated with high quality mineral oils plus selected additives, ideally suited to older technology and classic car engines.
Here are the specsgiven
SAE 20W-50
Test Method(s) Unit Typical
Density @ 15C, Relative ASTM D4052 0.884
Viscosity, Kinematic 40C ASTM D445 mm²/s 157.71
Viscosity, Kinematic 100C ASTM D445 mm²/s 17.3
Viscosity Index ASTM D2270 None 120
Pour Point ASTM D97 °C -27
Zinc, % wt ASTM D4951 % wt 0.1
Calcium, % wt ASTM D4951 % wt 0.13
Flash Point, PMCC ASTM D93 °C 200
Total Base Number, TBN ASTM D2896 mg KOH/g 5.0
this is a good web site under the Ireland section that discusses a lot of classic car oil issues.
http://www.castrol.com/castrol/subsection.do?categoryId=9014105&contentId=7027417