Could it be:

Running too low of an octane causes pre-ignition. Pre-ignition get's everything hot, especially the pistons and walls. Getting everything hot causes the oil to heat up as well. Hot oil gets thinner, and the oil pressure drops.

What was octane in 1969? Like 100? Plus the gas was leaded too. I could imagine it actually being tough to keep that old of an engine running in good shape on "today's gas".

???