[M.D. Holloway]

Here is something one of my guys wrote a while back. It is decent and provides some good insight. Take a look:

"Engine sludge is a thick, jelly-like substance that is detrimental to the performance and extended life of an engine. Sludge obstructs oil passages and restricts oil flow. Once built up, it reduces heat transfer, increases the operating temperature and hampers engine operation. Sludge will ultimately lead to shortened engine life.
Although the oil appears to be at fault, it is actually the victim of mechanical and chemical attack. The formation of sludge is a complex interaction of components. Each of the following
factors deserves attention.
SOOT: Soot is fine powder that is a product of incomplete combustion. This carbon substance enters the crankcase with exhaust blow-by gases that escape past the piston rings. Since soot is a very fine powder, it thickens oil by a process called “soot loading.” It gels the oil like a cake mix thickens milk. If your motor oil becomes excessively thick, there will be less oil circulated through the engine. Also, the oil will leave a thicker oil film on the engine parts, which prevents proper heat transfer. By remaining on the hot parts, the oil will burn and form deposits.
HEAT: Engine heat, a natural result of internal combustion, takes its toll on your motor oil. In the presence of air, oil undergoes a process called oxidation, which becomes more severe as the temperature increases. Oxidation thickens the oil and produces corrosive acids. Left unchecked, your oil would degrade into a tar-like mess.
While you want your engine temperature above 210ºF to evaporate unwanted contaminants, above 250ºF the oil is more prone to oxidation. At temperatures of 300ºF, this process occurs rapidly. Oil companies have some additives that contain powerful oxidation inhibitors. These additives break down the oxidation cycle as shown in Figure 1.
As long as these inhibitors are present, no significant oxidation will occur. However, these additives are consumed with time. After their depletion, oil oxidation proceeds rapidly. Regular oil changes are needed to remove the unwanted products of combustion and to replenish the supply of oxidation inhibitors.
FUEL: Fuel enters your crankcase with exhaust blow-by gases in unburned and partially burned forms. It is chemically unstable; therefore, it reacts with itself and the oil to form gums, varnishes and asphaltic-type compounds. These resinous substances are also unstable and react further to cause even more oil thickening.
WATER: Water gets into your crankcase typically through condensation or in exhaust gases that escape past the piston rings. It’s your engine’s job to get rid of this moisture by operating at sufficiently high temperatures. However, all engines operate periodically at low temps and experience some water contamination. When this occurs, water becomes emulsified. That is, it is absorbed by the oil, which thickens the lubricant. As a result, the oil does not flow or cool well. The increased viscosity can cause the oil to burn, creating engine deposits. Unfortunately, there is not much that engine oil can do to reduce the harmful effects of water. Oil changes every 3000 miles are the best way to take care of this problem.
ACIDS: When fuel burns, some products of combustion react with moisture in the system to form acids. These include sulfuric, hydrochloric and organic acids. Sulfur-based acids are undesirable because they attack the oil, reducing its detergency. Organic acids react with unburned fuel to promote sludge and varnish. In addition, acids can cause additive settling, or dropout.
DIRT: People associate dirt with engine wear. It can also play a role in sludge formation. Wear of piston rings and cylinder walls causes an increase in piston blow-by.
Since exhaust gases contain harmful by-products, their presence in the crankcase should be minimized. Some of these compounds will escape past the rings.
ENGINE COOLANT (Antifreeze): Coolant is your engine oil’s number one enemy. Engine sludge is inevitable when oil meets engine coolant. Contamination of your oil with coolant promotes sludge by two means. First, it introduces water into the oil. This presents problems that we’ve previously discussed. Second, it brings into contact oil and coolant, which are incompatible fluids.
Oil and coolant react to form deposits as they experience temperature changes in your engine. Some are gooey or gel-like. Others are hard, brittle deposits that plug oil passageways, reducing oil flow. These two types of deposits guarantee a shortened life for your engine.
No oil additives available will help solve this problem. The only solution is to drain the oil and locate the source of contamination. Then, have the mechanical problem repaired. These are the enemies of your engine oil — Soot, Heat, Fuel, Water, Acid, Dirt & Engine Coolant.
Sludge formation is the result of one or more of these factors:
• severe service driving with improper drain intervals
• mechanical malfunctions
• inadequate engine maintenance
Severe Service Driving: The term “severe service” refers to:
1. Short Trip/Stop & Go
• going to the corner store
• driving in crowded downtown
2. Extended Idling
• sitting in traffic
• delivery truck operation
3. High temperature operation
• low speed driving at high
• ambient temperatures
4. Extreme Cold
• starting engine below 0˜F
5. Heavy Loads
• operating in hilly regions
• trailer Towing
6. Dusty Conditions
• more common than expected
Although some of these situations may not seem severe, they all put additional demands on your motor oil. Under these conditions, automakers require that drain intervals be reduced to three (3) months or 3000 miles. Compare this to “normal” drain intervals of 6000 to 12000 miles. Severe conditions are not uncommon. It is estimated that we operate our vehicles 80% of the time under severe service. Therefore, it is wise to establish 3000 mile or three (3) month drain intervals to assure proper maintenance unless you are sure that you’re driving under the normal category.
Mechanical Malfunctions: A flat tire, engine knock or an electrical problem is easily identified. However, a small, subtle malfunction, like a leak from your cooling system into your crankcase, can create big problems for your engine. The resulting oil and coolant mixture reacts to form harmful deposits. An improperly operating cooling system will cause problems, too. If your thermostat sticks and does not allow coolant flow when needed, your engine will run hot. You might not even notice the difference on your temp gauge. Constant elevated temperatures promote oil thickening, after thousands of miles, this can create sludge.
A clogged or defective PCV valve can contribute to sludge formation. If this valve does not operate properly, harmful exhaust gases remain in the crankcase. These gases, which contain water, acids, soot, etc., promote sludge.
Poor Engine Maintenance: Lack of engine maintenance is probably responsible for more sludged engines than all other categories. Establish proper drain intervals for your type of driving. A timely oil change interval of 3,000 miles or every 3 months is inexpensive insurance for your car’s engine.
Use the proper quality oil for your engine. Never use oil that is rated less than the minimum API Service Category specified by your car manufacturer. To assure yourself proper quality, use oil that is rated API SJ or contains the ILSAC “starburst” symbol on the product label. This symbol assures that the oil has been tested and certified to meeting minimum requirements.
Finally, the cooling system must be flushed and refilled periodically with a proper water/coolant mixture to prevent engine overheating and offer optimum engine performance.
Some other conditions that promote sludge are: clogged air filter, low oil level, low coolant level, bad fuel, etc.
Often, the oil seems to be at fault. But more often, oil is the victim of mechanical malfunctions, extended oil drains or a poorly tuned engine. What can you do to make sure that sludge won’t drive your engine to an early grave?
• Change your oil and filter every 3000 miles or three (3) months, whichever is sooner.
• Maintain your vehicle in good operating condition.
• Make sure your car is well tuned. Efficient, lean combustion produces fewer harmful acids, soot, unburned fuel than a rough running engine.
• Be aware of unusual indicators like high oil pressure, gummy oil on the dipstick, a flooded valve deck or excessive exhaust smoke.
• Use an API SJ, high quality motor oil or the latsest API spec oil.