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In a nutshell, sludge is old oil. "Gel" is apparently a different sort of problem, but one that has similar deleterious effects on the engine.
New oil, for those who might not know, is a clear honey-gold color, and pours like a light syrup. It's available in various brews and various viscosities (thicknesses), and intended for particular applications.
Oil's obvious function is to keep the many metal parts in your engine away from each other, so they don't scrape together and wear each other away. It does this by being forced between the parts while under pressure, that pressure being supplied by the oil pump. Those parts that can't be supplied by pressure are lubricated by the oil being splashed and slung around the motor by the squirting pressurized oil. The amount of oil between the parts is very thin, on the order of half the width of a human hair. Believe it or not, that's all that's needed.
Oil has other very important functions, however, that many people are not aware of, such as
cooling, sealing, cleaning and noise reduction.
In addition, automakers will sometimes take advantage of the oil's pressurized state to operate other parts inside the engine, such as valves and tensioners, which depend very heavily on clean oil. Honda's VTEC is a case in point. What has all this got to do with sludge? DIRT.
Engine oil gets dirty. An engine works by combining air and gasoline, then exploding that mixture. It does so the entire time it's running. In doing so, it produces a vast number of byproducts, ranging from chemicals to heat, water, soot, and even raw fuel.
Some of those waste byproducts end up inside the part of the engine where the oil is. This is normal and is impossible to stop completely. The problem is, the chemicals, water, fuel and soot get into the oil itself and start to interfere with its effectiveness. Modern oils are designed to cope with this for a given length of time. That length of time (plus a safety factor) is known as the oil change interval, and is different from model to model and from driving style to driving style. Your Owner's Manual will specify the correct interval for your car. Honda's newest models specify 10,000 change intervals, even for the car's very first change. If you're not sure, or want to play it safe and don't mind spending a bit of extra money, oil and filter changes every 3K to 5K miles or every three months is a safe default, especially in climates with snowy winters.
Modern engine oils are high in detergents, which strip contaminants off engine surfaces (exactly the same way dish detergent strips food off your dinner plates), and in dispersants, which trap and surround those contaminants so as to keep them from re-coating the newly-cleaned engine surfaces.
Oil can only clean and suspend for so long, then loses its ability to hold all that junk in suspension and begins to drop out the excess contaminants, which then begin covering the engine's parts with a gritty, gummy paste.
That gritty, gummy paste not only accelerates wear and causes the engine to run hotter than it should, but clogs up the valves and tensioners the maker may have been using the oil to operate. An oil change, performed in time, drains out all that gunk along with the old oil.
There is one exception to the above: Break-in oil. When your engine was built, the manufacturer may have installed special break-in oil that is different from the oil the engine will use for the rest of its life. I understand the main difference seems to be that break-in oil is low in dispersants. Why? I'm not sure. Your new engine has special requirements that used engines do not: A newly-built engine has rough surfaces that need to be smoothed off, and assembly detritus that needs to be flushed out. At the appropriate interval, which reasonably corresponds to the point that the engine's internal surfaces are most of the way to being their final textures, the oil and all its contaminants are drained out. Your owner's manual may even contain a specific instruction not to do the initial oil change too soon. Even so, an engine is not really fully fully broken in until about 5,000-10,000 miles have passed. I've been told that many manufacturers are no longer installing special break-in oil on their engines, but the same oils you would buy over the counter. Most now even come from the factory with part or full-synthetics, which was a no-no at one time.
Luckily for us, oil has improved vastly in the last twenty years. Sludging used to be a very common occurrence, and one that got much worse in a hurry with the new, higher performance, hotter-running engines that began to come to market in the '80s. That sludging was due to oxidation ("coking"), not contaminants. Oxidation is breakdown of the oil itself, and is commonly associated with high heat levels. Oil manufacturers work hard to prevent sludge and oil breakdown. Sludge is nowhere near the problem it used to be.
Failure to change your oil at the interval appropriate to your driving conditions is considered abuse. Most engines are fairly tolerant of some degree of abuse (such as 10,000 mile intervals on non-synthetic oils) without suffering catastrophic failure, even if they do accumulate some sludge as a result.
Unfortunately, there are a few engines on the market today that appear to be somewhat less tolerant of such neglect. Among them, Toyota's famous 1MZ-FE 3.0L V6, the 2.0L and 2.3L SAAB engines, and the 2.7L Chrysler V6.
Toyota, a manufacturer with a stellar reputation for quality and durability, has recently gone through a highly-publicized bad spot with its 3.0L 1MZ-FE V6 engine. This is the engine installed in the Camry, Sienna, Avalon, various Lexuses, and some light truck products. Some 3 million of these engines are on American roads, and about 0.1%, or about 3,000 engines have reportedly been affected. Toyota has already implemented an extended 8-year "gelling" warranty, with unlimited mileage. The only proviso is that the owner must be able to show evidence of having had oil changes done at timely intervals, which would simply be valid oil change receipts. Toyota is being fairly lax about the documentation rule, however, probably for publicity reasons, and is fixing engines even where the owner has no documentation at all.
I understand Toyota makes a distinction between "gelling" and sludge, honoring claims only on vehicles that are deemed to have "gel" and not sludge. "Gel" is supposed to be primarily caused by overheated oil, whereas sludge is primarily contaminants resulting from neglect.
I don't know about SAAB and Chrysler, but Toyota did make a small change to their engines to help them cope better with abuse. The change involved the baffling in the valve covers, the drain holes being enlarged to allow better oil drainage from the baffling even if the oil is thicker than it should be, and if it should become coked due to excess heat. Toyota has not changed the engine's head design since 1994, however.
The typical sludge story involves an engine that fails catastrophically at about 25-40,000 miles, with the owner unable to show proof that more that one (or maybe two) oil changes were done in that time. In more than a few cases, the failed engines were found to have the factory oil filter still attached, even though the owners swore up and down that they'd abided by the specified oil change interval.
There are a number of views on sludge, with one camp saying that sludged engines can't withstand abuse the way some other engines can, and others saying that it is solely owner neglect. The Usenet group alt.autos.toyota has had many lengthy and informative threads over the last several years on this subject. If you go to Google Groups and search for the terms toyota sludge, you will end up with probably a week's worth of reading.
Here are some sludge and oil links:
BobIsTheOilGuy
Chris Longhurst's Oil Bible
Vintage Triumph Register Oil Overview (good charts!)
Vintage Triumph Register's Lubricants Notes (from Redline)
Yotarepair
Canadian Driver
Nordic Group
Wolfgang's ML Page
Some high-end automakers, notably Mercedes, BMW, and Cadillac, have extended oil drain intervals far beyond those generally recommended for lesser cars. These intervals can reach as high as 20,000 miles. However, such systems also require very expensive synthetic oils and come with sophisticated oil monitoring devices. The engines usually have oil pan capacities much larger than cars that have shorter drain intervals. A Corolla engine may hold 3.3 quarts, a Mercedes engine up to 10 quarts. Capacity makes a big difference in oil life.
More links:
http://www.carseverything.com/content/previews/mercedes_benz/article/1253.11/
(Scroll down to the section called ASSYST).
http://autorepair.about.com/library/faqs/bl804a.htm
