Sludge can be the result of poor maintenance. But it can also be the sign of a more serious engine defect. Oil has two very important jobs to keep an engine running cool and efficiently. It keeps mechanical components lubricated to reduce fricition while absorbing any contaminates that would effect the engine's ability to run efficiently such as soot, dirt, water, metallic particles, and excess fuel.
When They're to Blame for Sludge
A defect in the crankcase ventilation system like the one found in the 1998–2005 DaimlerChrysler’s 2.7L engine. This engine is notorious for allowing hydrocarbons to enter the oil and break down the additives that help gather all the gunk.
If coolant gets into the oil the extra contaminates can severely degrade the oil’s life. One well known cause of this problem is when the water pump gasket fails leading to an incredible amount of sludgy complaints for the Concorde, Sebring, Intrepid and Stratus.
Engine temperatures are too hot causing early oxidation in the oil. The oxidation only gets worse when that heat lasts a long time. Volkswagen’s 1.8L turbocharged engines have a history of sludge-related complaints. From 1997-2005 their turbocharged engines came equipped with a tiny 3.7-quart oil capacity. And mechanics say it doesn’t provide the engine enough oil to dissipate the heat and it removes all margin of error for owners who may be a little late on their oil change.
When You're to Blame for Sludge
Not changing the oil regularily because there's only so much oil can do before it reaches its absortion capacity, where it thickens and loses its lubrication benefits. As that happens fricition gradually increases and generated heat. That heat continues to thicken the oil in a vicious cycle. More friction → more heat → more sludge → more friction → ∞. That's why it's imperative to follow your recommended oil change schedule. But it's also important to pay attention to your engine's specific consumption habits.
Just blindly following the manufacturer's recommendations is a sure-fire way to get into trouble. Especially if you own one of the cars with a history of problems. Automakers have said that owners should be checking their oil at every fill-up which is ridiculous. However, every few fill-ups it can't hurt to check the oil dipstick for volume and oil consistency.
Ignoring the signs of sludge like a check engine that does unchecked, a drop in oil pressure, a loss of acceleration, an engine stalling out, or scornful looks from mechanics.
FYI, some engines are bound to have sludge and that might be something worth knowing before you buy your next car.