There is a bypass valve built into a full flow filter housing that allows oil to route itself around the filter element in case the media gets severely clogged. The pressure differential is usually 5 pounds or less, so as long as the filter media is fairly free-flowing, most of the oil gets filtered even when the viscosity is high. Most full-flow filter elements have very thin pleated media with lots of area for the oil to flow through, so it must necessarily do a pretty poor job of filtering small particles out of the oil.
The manufacturers try to strain out anything like a metal fragment that could become wedged between a bearing and a crankshaft, for instance. Soot and small dirt particles that do not present a danger to moving parts just don't get removed from the oil flow. The detergent/dispersants in detergent oil grab these contaminants and hold them in suspension until they can be drained out during an oil change.
Jerry
