As someone who has used Castrol 20w-50 for the past 30 years in my '62 Giulia spider, I have been following this thread with interest.
I did some Google research and found that on the Castrol UK website they have a listing under classic engine oils for a XL 20w-50 which they claim has all the additives necessary for pre 80s eingines.
I tried to contact Castrol UK to find out if it is available in the states but so far have not heard back.
1962 Giulia spider