Oil sludge is the result of the breakdown of over-stressed oil in your engine and it is becoming extremely common in all vehicles - in particular, those manufactured from 1997 onwards.
Oil sludge is like a gel that sticks to your engine parts and blocks oil galleries preventing the normal flow of oil throughout the engine. Normal oil flow provides maximum wear protection and aids with the cooling of the engine. Sludge actually stores heat rather than releasing it, putting added stress on the radiator and cooling system. It also reduces oil pressure and fuel economy and will lead to component wear and potential failure of expensive components such as timing belts, idle controls, etc.
Unfortunately there is no one simple cause of sludging. Contributing factors include:
Things such as engine design, sump capacity, cooling system efficiency, etc are beyond your control. The things you can influence are the service intervals, the maintenance procedures, and the selection of the oil that is used in your vehicle.
Extended oil drain intervals are false economy. It is vital that you tailor the oil change interval to the type of driving done by your vehicle. A 10,000 km oil drain using good quality oil may be fine for a vehicle doing regular long distance driving but may be very bad news for a vehicle doing short distance, stop start driving or one that does less than 10,000 km per annum. Such vehicles should be serviced using quality oils at a maximum of 7,000 kms or every six months, which-ever comes sooner.
Whether you change the oil yourself or use a qualified mechanic to do it for you, it is imperative that the engine is warmed before the old oil is drained out. Failure to do this will definitely leave old oil and contaminants in the engine which will only further deteriorate throughout the next oil drain period.
Using the wrong grade, quality, or viscosity of oil will certainly contribute to oil sludging. Once again, saving a few dollars by using an inferior brand or grade of oil will inevitably cause greater costs in the long run. Some companies argue that the use of a full synthetic oil will reduce the incidence of sludge caused by high temperature operation. This is certainly true of a full synthetic oil from a reputable company but we must remember that it may not eliminate sludging caused by other factors.
In simple terms, while no-one can guarantee your car will never suffer from problems caused by oil sludge, you can minimise the risk by tailoring your service intervals to suit the way your vehicle is driven, change oil, oil filter and air filter at every service, use the correct grade of oil for your vehicle from a reputable manufacturer, and if you have your car serviced for you, select a reputable mechanic such as a Repco Authorised Service Centre, who use quality oils and follow correct procedures.