Originally Posted by RDY4WAR
That's something I hadn't thought about before either. Thanks for bringing it up.
Knowing now the role of "detergents" in oil, and the lack of abrasiveness, what additive, base oil, etc... is preferred when trying to clean out a dirty, sludged up engine? Do the detergents like Ca and Mg help break down sludge and carry it to the filter or do other additives fill that role? Are there no additives used that are designed for such a role? Are flushes and solvents like MMO the best option for such a scenario?
I'm probably going to upset a lot of people now by saying there in NO OIL ON EARTH that can clean out a properly dirty, long time sludged up engine!
100% of the work that you put into designing an engine oil is focused towards PREVENTING problems. Primarily you try to restrict the degree to which the oil oxidises because so many 'real-life' issues stem from this (out-of-control viscosity increase, corrosive metal loss, sludge deposition, etc). Under normal circumstances (ie API/ACEA/OEM testing) you put zero effort into thinking how an oil might 'correct' a severe problem, after the event.
There are engine oils out there that claim 'clean-up' for advertising/product differentiation reasons. Invariably this uses a non-stardardised in-house test which you should always treat with caution. The test usually involves an engine 'dirty-up' phase (on a very bad oil) followed by a 'clean-up' phase on a new candidate oil. And yes, you can demonstrate a degree of sludge clean-up to support a claim.
However, in viewing these results, you should bear in mind several things. During the 'dirty-up' phase, the deposition of sludge occurs over a few hours at the end-of-test. You purposely monitor this to stop it, before it gets too bad. You then, as quickly as possible, drain the dirty oil, add the new oil and restart the engine for the 'clean-up' phase. You typically find that 'clean-up' starts straight away.
The flaw in this approach is that unlike in real-life, the relatively light coating of deposited sludge has no time to 'bake on' and compact down. This makes it relatively easy for the fresh candidate oil to remove what is relatively accessible, 'fresh' sludge. My guess is that oils don't have to be particularly special to 'clean-up' sludge in this way & any modern oil would do equally well.
Ironically, as oils have improved & their ability to PREVENT problems has moved on by leaps & bounds, their ability to RECTIFY problems such as sludge has probably fallen off a cliff. If you're looking to resolublise sludge, what you chemically want are base oils rich in heavy aromatics & hetroatoms (Sulphur, Nitrogen & Oxygen). These are now absent in Group II & beyond.
Ideally, to get rid of 'proper' sludge, you might give your engine a nice, long soak in hot Carbon Disulphide. However this might very well kill you, so kids, please don't try this at home! I sometimes muse about what would happen if you ran your sludged up engine on an oil based on Naphthenic base oil? Naphthenics have very low Viscosity Index & are hopelessly unstable. However they are very rich in aromatics so running one for say a week max (then dump & flush) might be something you could get away with. Alternatively (and this is strictly for the birds!) why not see what you could do with the 'extract' you get from Furfural Solvent Extraction (a process used to make Group I base oil). This is the grot that no-one wants but it's probably the perfect solvent for thick sludge!