There’s a whole host of factors that would determine the “ideal” oil to use, making this a simple question with a complex answer.
A ‘67 Dart could have anything from a 170 cube slant six to a 383-4 engine, but, surprisingly, the displacement has little effect on the oil I’d recommend.
The “theory” behind viscosity selection has also changed quite a bit in the last 40-50 years. It was thought “back in the day” that a heavy viscosity oil was needed for high performance protection, that view has gradually fallen by the wayside. Except for a very few specialized cases, the heaviest oil I’d use on a street-driven car is 10W40. In anything but a southern summer, 10W30 would be “it”. I’d not go thinner, even though lighter oils do build initial pressure more quickly, the fact remains that carbureted engines pollute and dilute the oil more than modern SMPI setups; the heavier viscosity helps protect the bottom end. Unless we’re talking an Alaskan or Canadian winter, avoid anything lighter. And drag race engines, which see higher RPM and greater specific outputs, now use even far lighter oils, mainly to reduce windage losses. Windage is real nasty — picture the crankshaft’s counterweights slamming into the oil at high RPM: This friction heats the oil and actually (mechanically) breaks it down. Light oils reduce this effect significantly.
Synthetics offer some advantages, whether these are worth the premium price changed is debatable. They thicken less in lower temperatures, insuring fast pressure build-up on that cold start. They also break down far less at very high temperatures.
Personally, I use the cheapest synthetic 10W30 in just about everything. Currently, that translates to Wal-Mart house brand (SuperTech) full synthetic 5-quart jugs.
Remember, however, that description of oil dilution. Carbureted car’s should have shorter drain intervals than modern cars. If the car is regularly warmed up and “hammered”, and the carburetor has stock calibration (not over-rich), you might go 3,000 miles — no more. If you have a pig-rich aftermarket carb, and only use the car to cruise 10 miles to the local cruise-in, I’d revert to time-based drain intervals: 90 days maximum.
One tip that’s helped me achieve long life from all my engines: Always pre-fill the filter with oil before installation.