Prior to about the mid 1950’s, motor oil did not have the modern detergent additive we have today. Therefore this type engine oil didn’t keep dirt particles in suspension. Foreign material would slowly settle to the bottom of the oil pan and on other flat surfaces such as the valley of the head.
If you ever removed an oil pan from a 60 year old engine that has been setting, chances are good there can be up to 1/4 inch of sludge in the bottom. This is not a bad thing! The non detergent oil has allowed the dirt to settle. As most 60 year old engines did not have an oil filter, this dirt settling was what should happen to increase the engine life.
The problem begins when a new owner wants to get his old vehicle running. He installs plugs, points, back-flushes the cooling system and changes oil. Oops, he probably adds detergent motor oil! The dirt deposits now begin to slowly go back into suspension and your engine life is shortened. Unfortunately with no oil filter the dirt stays in the oil and shortens the remaining life of the engine.