65Convertible: Here's my best attempt at a "horse's mouth" answer. From my "1973 Petersen's General Auto Repair Manual," recommended oil weights (including a few obsolete weights) for typical rides of that era:
CONSISTENTLY ABOVE 32F DEGREES:
SAE 30w, 40w, 10w-30, or 20w-40
0 DEGREES F TO 32F:
SAE 10w, 20w, 10w-30, 10w-40
BELOW 0 DEGREES F:
SAE 5w, 10w, 5w-10, 5w-20
Interestingly, as I mentioned before, 20w-50 isn't even mentioned. I'm curious as to when that weight became popular. In addition to having different seal swelling characteristics, the "high mileage" oils tend to use better base stocks as well, and are generally superior to standard oils. But keep in mind that ANY basic API "SL" oil is LIGHT YEARS ahead of what that car came with from the factory, so even the 59 cent Shell Formula somebody mentioned in another thread would be a perfectly good choice. Whether to go "hi mileage" is simply a pocketbook issue for ya -- there's no right or wrong answer, although if you've got some leaky seals, that might affect your decision. Personally, I'd opt for a low-cost 10w-40 year-round in Georgia.