Straight 30-weight oils are often specified for use in air-cooled engines because they don't have any Viscosity Index Improvers
(VIIs), which can Shear Down
under high temperature and load. VIIs are polymer additives that increase an oil's viscosity as its temperature increases.
Multi-grade dino (conventional) oils start off as a light weight oil (like a 10W oil in a 10W-30). Over time, the hot viscosity of a 10W-30 dino oil decreases because the VIIs become shorter. In contrast, a synthetic 10W-30 starts off as a 30-weight oil which is manufactured to have the viscosity of a 10W oil at cold temperatures. A synthetic oil uses very little, if any, VIIs and is therefore much more shear-stable than a dino oil.
Modern 10W-30 oils are a lot better than what was available in 1967 so you should not have any problems using a 10W-30 dino oil. Since your engine has a flat tappet cam and minimal emission controls, you would be better off using a Heavy Duty Engine Oil (HDEO) because they have stronger anti-wear and detergent/dispersant additives than a Starburst oil
Although 10W-30 HDEOs are available, I would also consider using a synthetic 30-weight HDEO. However, due to their ability to flow in extreme cold, 30-weight HDEOs are usually found as 0W-30 and 5W-30. Because of their high viscosity indexes, 0W-30 oils will not thin out as quickly as a straight SAE 30, even in Furnace Creek in July.
See the ACCCC Engine Oil Article
for more information.
For a list of 30-weight HDEOs, see Heavy Duty Engine Oils