Engines build up sludge over time from the by-products of combustion mixed with old burnt oil.  Sludge is a black tarry substance which leaves baked-on deposits on the working surfaces of your engine and blocks oil-ways, eventually starving them of lubrication and, in the worst case, causing engine failure ("black death").  The causes of sludge are varied:  hard, fast driving leading to high under-bonnet-temperatures combined with the use of mineral oils; lack of maintenance, particularly failure to change the engine oil within 20000 miles; city driving - stop/go starting; an abundance of cold starts/short runs; engine overheating.  If your engine has been poorly maintained, cleaning it with a mild flushing agent will release all this sludge which will get caught in the oil filter.  Change the filter when you fill up with fresh oil.  We recommend using a mild proprietory flushing agent (do  NOT use neat Kerosene, which can have an adverse effect on turbos and turbo seals), or you can use automatic transmission fluid (ATF) as a flushing agent.  ATF is a low viscosity oil containing a high level of detergency and is much less aggressive than many proprietory engine flush treatments.  Use 1 part ATF to 3 parts cheap standard motor oil and run the engine for about 20 minutes.  Release the oil and ATF from the sump and leave it to drain for a good 10-15 minutes, just to make sure all the sludge and any other deposits have drained from the engine.  Discard the old oil filter.  Fill to the correct level with a good quality motor oil of a type recommended for your car and fuel type, and fit a new oil filter. 

We do not recommend back-flushing using power flushing equipment - this highly aggressive flushing method reverses the flow of the oil through the engine to loosen sludge, but can actually force the sludge and deposits into oil channels and valves, blocking them and leading to engine seizure.

For more information on sludge and its effects on your engine consult 'the engine oil bible' on the internet.