Synthetic oil was first used in aviation jet motors by Germany in WW2. The early synthetic from the 60ties and 70ties had a different swell ratio for the seals then the mineral oil, hence some of the early problem. Too fast and the seals are tight, too slow and they leek. Change back to mineral and problems again.
All that is history.
Mineral oil, the result of refining the oil we pump from the ground is a natural product and as such has variations in it's molecular structure according to provenience. Mineral oil is susceptible to heat and above 116C (240F)*it brakes down and oxidises forming varnish and sludge. The natural paraffin contained in oil solidifies at low temperature making circulation at cold start difficult. Yet mineral oil has been used as lubricant for a long time and it does provides adequate lubrication.
Synthetic oil is the result of chemical reactions, synthesis of esters and other nice stuff. Their molecules are smaller and much more resistant to heat and offer a far superior lubrication. They do cost more but they do a much better job for cold startup, no sludge, no varnish, no degradation when hot. On this point you may think my oil never get's 115C (240F) since engine is usually 88C right? Wrong, the oil secondary job besides lubrication is heat extraction from the hottest points, namely the inside of pistons, head and cylinders and it gets very hot there. It cools down in the heat exchanger and in the sump but it gets hit by burst of very high heat that slowly destroy the mineral oil.
Synthetic oil is a superior product and to say it is unnecessary or a luxury in a diesel that may be old and used on weekend is misguided. When the best situation for an engine is working 24/7, the worst is working*a few*hours once a week and if an engine is working in the worst possible situation, synthetic oil may just be what is needed to keep it together.
As for super-filtration with the toilet paper bypass filter, I think that the suppliers of such superior filters would be best served if they stick to what they know, namely filtration. Their products are excellent secondary filters and do extract small particles that harm the engine and that a direct screw on filter can not be made to retain. In order for a direct filtration filter to retain one micron particles it would have to be 20 times bigger to allow the oil to circulate at all temperatures.
Yet the makers of bypass filters insist in bringing up this nonsensical mumbo jumbo of the eternal oil that does never need changing. Clearly a big fat lie that does not serve them. Yet an additional bypass oil filter, providing is not installed to prolong oil change is one way to keep the oil free of harmful solids. Keep your oil always as new...who does not want that?
Can synthetic oil quite down an engine?*
Perhaps the right question is: Would appropriate lubrication quite down an engine? And the answer is of course yes. An engine running on thick oxidised and* contaminated mineral oil will rattle and bang and when oil is replaced, particularly if with the appropriate synthetic oil that is more fluid at start up it will appear that the new oil is the reason for the engine running smoother. Of course good quality new mineral oil may have just about the same effect*