OK...gotta be really careful here (RE: sludge):
just like Octane rating in gasoline, sludge is an overly generalised term for oil-related deposits inside a 4-cycle internal combustion engine. General pubic perceived anything from visual coatings (most of them call this varnish but I have seen some autoparts/mechanics insisting in calling it "sludge" to some serious hard, cabon-like deposits on the upper portion of the valve train section.
With the introduction of PCV system (aka "positive crankcase ventilation"), the use of unleaded gasoline, the constant technical advancement in power output(efficiency), emissions control and environment-related pollution reduction, almost all gasoline engines with emission controls runs hotter than before, requires to push the power output and emissions envelope far beyond what automotive engineers would have never imagined possible some 20~30yrs ago.
That being said, however, with all these technological advancements and emissions standards come a whole new kind of ballgame (or issues, for that matter), and that is the accelerated deterioriation of engine lubricant and different requirements in terms of maintaining a wide thermal operating range, cleanliness, valvetrain wear, AW/EP/Anti-scuffing properties, anti-foaming, longer oil change/servicing duration, ability to absorb/handle moisture , exhaust gas from ring leaks, burnt fuel additives byproducts, gasoline-related byproducts, oil dilution caused by cold-starts, etc.as a form of combustion byproducts, and so much more.
Sludge are normally associated to 1 or more of the aforementioned issues and sometimes bad gasoline/motor oil chemistry as well. It's dark in colour and mushy texture, and sometimes depending the how long the issue has been, it can becomes crusty dark/black in colour.
Modern latest automobile approved API/ILSAC graded multi-viscosity motor oils are very good in controlling the formation of sludge, even if you choose to go with proper API rated off-da-shelf supermarket no-name motor oil. That being said, however, most automobile users/owners nowadays tend to abuse their vehicle far more often by neglecting one or more of the following:
-tuneups. Proper ignition timing and top-notch fuel delivery and mixing will ensure optimal power output, minimal(per design specifications) exhaust gas byproducts, and minimal oil dilution due to fuel injector related problems. A weak spark is pretty much as evil as a slightly bad injectors IMHO.
-missing proper PCV servicing. PCV is designed to ensure proper internal venting and recirculation of combustion bypass gas, evaporated fuel fumes from hot motor oil, trapped moisture, etc. goes back into the combustion chamber to be re-burn again (2nd time round). Given the rather hot running engine design these days, the tendency of a clogged PCV system is very, very high and frequent. Regrettably, most mechanics I spoke to inc. stealerships tend to ignore this part (and I've been giving them #@$%!).
-skipped oil maintenance or totally disregarded OCI service. Yes, even though most car manufacturers claimed motor oil servicing to be good for 5000+miles OCI (with decent API/ILSAC rated motor oil), the trouble is, I've seen over 2/3 of auto owners tend to drive past 10,000miles before they change out their already severely oxidised, additive-depleted motor oil. Some of them consistently do so on a regular bases, and when their cars starting to smoke due to stuck oil control rings, they lay blame to the car manufacturers....(you know the rest of the story).
-inferior/inconsistent bulk motor oil used in most OC chains and stealerships. While these are mostly properly API rated, with mainly Gp 1 or Gp 1+2 blend in mineral oil grade, I recall from Molakule or Terry Dyson that these oils have just barely met the API requirements and they envelopes tend to go far quicker than brand name proper API rating and grade brand name motor oil. Guess what? next thing you know, with that evil 10,000miles skipped oil changes, you'll get sludge, stuck oil control rings and/or accelerated valve stem/seal wear.
IMHO sludge is a culmination of
a)oxidized motor oil base (insolubles),(b)combustion byproducts; (c)clumming of the supposedly suspended dirt particles, carbon byproducts, PCV byproducts, moisture, etc., with (a) being the biggest contributor of all. All properly, chemically-balanced multi-vis API multigrade motor can stay clean/combat sludge formation so long as the aforementioned practices have been rigidly observed.
IMHO you have to be more specific as to what you mean by "well cared for from now on out". Do you mean cosmetically? or mechanically (inc. conservative OCI with quality motor oil)? If latter is what you referring to, there shall not be the possibility of sludge forming.
why will engine lasts longer if sludge free than if it's left with sludge? Simple.
Consider sludge is the "consequence" to the cause of maintenance and engine servicing neglect (cause), a sludged up engine is a sure sign of engine neglect and when you see that, it just means that the engine is on it's way to some serious servicing, or overhaul/rebuild it what I would refer to.
A properly maintained engine with proper API rating should have 0 problems maintaining it's sludge free status throughout it's entire service life.
