More than Sticky Stuff
Petrol engines: Synthetics are magic. Not for old engines, but best on new engines after running-in has finished.
Diesel engines: Synthetics glaze cylinder bores unless you’re on the throttle hard all the time. Diesels have their own problems with oils.
As the piston goes up and down the oil lubricates it and helps remove some heat. The oil does two important things. First, it creates a friction loss between the cylinder and the piston by literally getting in the way and holding the two metals apart. Second, it washes away carbon that the explosion (fuel igniting on top of the piston) forces around the piston rings and down the side of the piston. Carbon is abrasive and must be controlled. A diesel engine makes more carbon than a petrol engine, so it needs more detergent in the oil to get the carbon into solution. Once in solution the carbon is removed by the filter.
The basic criteria for a good diesel oil is its ability to manage a lot of carbon. When the oil is fracted from crude oil, it mostly becomes acidic (bad for engines), so more sulphate ash is added to make the oil ph neutral. The more sulphate ash already dissolved into the oil, the less carbon the oil can take into solution.
If someone made engine oil from the small amount of oil that comes out of the fractionation column ph neutral, the oil would work much more efficiently at absorbing the carbon and generally have a longer life. Provided the other necessary cocktail of additives lasted and the oil maintained its viscosity (high temperatures will damage the oil and , especially, lower its viscosity) you could extend the oil change intervals. Oil labs can determine if the oil is still healthy, though the oil filter still needs to be changed out at the engine manufacturers intervals. If the oil sample from my Cruiser comes back indicating that the oil is healthy, I will stretch my next oil change intervals from 10,000km to 15,000kn.
Why? Because it’s saving me money.