Most modern oils get thrown away because of contamination by carbon (soot), unburned fuel or water (a by-product of combustion). Some readers may recall the ancient rituals of de-coking (on four strokes !) and "flushing oils". These were necessary evils to rid an engine of excess deposits. Crud built up around the piston rings, and valve stems and in the smaller oilways and passages. Some of this crud was due to poor combustion, some to fuel impurities, some to the relatively poor performance of early multigrade oils. One major source was the comparatively low operating temperature of older engines (poor thermal efficiency): to this day, an air-cooled engine gives its oil a harder time than a water-cooled one. This is often not because the oil is getting too hot, but because it is not running hot enough to burn properly or to evaporate the water formed by burning petrol in air.
These deposits cause the need for de-coking and flushing. You either mechanically scraped and polished them off the pistons or "washed" them out of the narrow passages with a high-detergent oil. This "flushing oil" was very good at getting out deposits, but a poor lubricant, so it couldn't be left in with the vehicle on the road.
Engines still make all these deposits, but the rituals are mostly dead and gone. Why? Because modern oils contain much more robust detergents, all through their service life they absorb the carbon and unburned petrol and water and acids and lock them in with nice safe, inert detergent molecules: protecting your engine. In passing, all these contaminants are far, far smaller than the particles the oil filter is designed to trap.
That is why we need to change the oil: to get rid of the contaminants, not because the oil has degraded in any way. A given volume of oil can only hold a given volume of contaminants, after that all the detergent molecules are busy and the contaminants start to settle on (and attack) the engine. Even if a very expensive oil can trap a bigger amount of crud per litre of oil (and the difference is marginal at best), a sump full of new, clean, unpolluted oil is the best way of ensuring a clean engine.
Something to beware of when using flushing oils, as they are not supposed to be run in the engine for a long time, if there are a lot of deposits in the engine and sump areas it will soften it only and when you put new oil in, it can then break away in lumps which can be big enough to block vital oilways. On cars with hydraulic tappets the flushing oil will get thru to the oil chamber but how long will it remain there before it is replace with good oil?, because when you drain the sump, oil remains in the tappets, also in numerous other places in the engine.
I would never recommend using any type of flushing oil, but if you do be prepared to a couple of cheap oil changes afterwards before putting in the good oil.
This is of course IMOA