That is a great site. Thanks for the link.
A few comments.
Cold starting, not long engine life was the priority, since they had 6-volt systems and after WWII, car engines were not expected to last very long, maybe 50-60,000 miles.
Acid and clay treated oils, not even close to todays Group I, had so much residual wax that they did not flow well cold. Adding kerosene to the crankcase was "homebrew" solvent refining.
Chevrolet engines back then did not have full-pressure lubrication. The rod bearings were lubed with a "dipper" scoop on the bottom. Thick oils probably dont work at all with that system.
Operating oil temperatures were lower in these low rpm low horsepower (by todays standard) engines . If you look at one of the oil specification charts, the viscosity was measured at 138F, not the 210F that is the SAE standard. True operating temperature viscosity of SAE 20 at 138F may be comparable to SAE 40 at 210F.