I think a couple of reasons are the lower power densities, the larger sump capacities, the short oil change intervals, and larger cooling capacities. The oils certainly weren't the quality or purity of today's oils, nor did they have the latest additive technology. A huge all cast iron/steel engine was required to put out less horsepower for the weight. Today in the US, we're running engines with high power densities, small radiators, lower sump capacities, and trying to circumvent problems with newer oils and additive packages to gain EPA approval at the same time. I see the EPA as a two-edged sword: 1. They foster competition for more fuel efficient engines with less pollution, 2. But they base so much of their regulatory edicts on pseudoscience.