I think it's primarily the detergents that do the cleaning, judging from the 'ashless dispersant' aircraft oils. From what I understand they haven't used detergents in aircraft oils since the 50s or so, as you don't want an oil dislodging a big plug of sludge and clogging an oil line when you're at 20k feet, but you don't want sludge to build either so they try to keep it suspended. Base stock and such differences seems like it might do some cleaning, as when sludge and varnish form it's by definition not soluable in that oil, but when using a synthetic after dino has been used there seems to be some cleaning. It's also interesting that they don't use 100% synthetic oil in piston engined aircraft as they found that they needed to add quite a bit of dino to get the cleaning/lead removal that they needed, and ended up with a blend that was essentially no different than existing blends. HDEOs will have more aggressive cleaning than PCMOs, some say too much as there is competition at metal sites for detergents, moly friction reduction additives (they don't seem to reduce wear) and anti-wear additives, but judging from the junk that people find in their filters even after exclusive Mobil 1 PCMO use I don't think that there is too much. Diesels seem to usually do fine with engine life. I like using synthetic/dino blends anyway for cost reasons, using 2 qts Mobil 1 T&S with 3 qts of Mobil 10W-30 in both cars, and 1 gal of Mobil 1 T&S with 2 gal of Delvac 1300 in the truck. Maybe I've ended up with optimum cleaning by accident.