With wide open valve covers with a crankcase under vacuum from the downdraft tube, and driving on dirt roads before the interstate highway system was created, an oil filter which barely sips a little oil at a time, couldn't possibly keep up with the amount of dirt entering the engine.
Most of the crud ends up in the bottom of the oil pan where it stays put. Back when these trucks were originally in service, this was all cleaned out when the regularly scheduled crank and rod bearing service was performed.
