Oil + additive viscosity

Stress is the load (force) per unit area and strain is the deformation of the structure in response to stress. So, the stress and strain in the valve train should not vary with the grade of oil used.

I think you are more concerned with engine wear and this depends upon the lubrication regime that the engine is experiencing and this is dependent upon several factors including RPM. ZDDP in the oil creates a sacrificial wear layer that prevents wear in the Boundary Lubrication regime. However, the ZDDP layer has molecular thickness and greater concentrations of ZDDP do not increase its protection but instead extends the oil's useful lifespan. Excessive ZDDP levels (over 1400 ppm of phosphorus - the antiwear component of ZDDP) are detrimental. See Engine Wear and Engine Oil Myths.

Although a higher viscosity oil will have a thicker hydrodynamic wedge, using an oil that is too thick will cause more oil to bypass back to the sump without ever reaching the bearings and valve train.

The peppier feel of your engine with 15W-40 compared with 20W-60 means that the thicker oil is wasting power through parasitic fluid friction. The additional fluid friction of higher viscosity oils also tends to cause the oil to run hotter.

Here's a discussion about oil for old Mopar engines: BITOG: What oil for 1968 Mopar?