Older engines rely on the fine sediment settling on the bottom of the sump and in nooks and crannies, because the oil filters of the day weren't capable of filtering out the fine particles without clogging.
If you go pre-war (WW2) few engines had oil filters at all. For those engines a basic straight 30 oil is better than even a basic multigrade. I believe that the demand for reliability in the field of war probably prompted the development of oil filtration, which was of course continued post war. 50's and 60's engines had an average 'life expectancy' of from 70 to 100 thousand miles, sometimes much less. I have had several cars in the last 30 years which have gone well beyond 250,000 miles untouched and with no special effort.
Modern oils keep the micro particles in suspension to allow them to be removed by the much more effective modern filters we have today. Another factor is the greatly improved metallurgy; the wearing parts of modern engines are greatly improved both in terms of wear resistance but also in terms of finish, the tolerances in modern engines can be much closer due to CNC machining and improved manufacturing techniques.
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