There is hardly any need for engine flushing if oil change interval is kept at strictly 5000 km. If flushing is at all needed it is with the purpose of getting rid of sludge and combustion waste. Tufoil inventor Mr. Reick has likened the engine to a "chemical sewer" - you know what a sewer is.
The problem is that after flushing the internal parts and bearings become " gersang" - dry like bone, think of old zinc roofs. Surface friction tends to increase and you are likely to get increased wear. The addition of Tufoil helps to restore the lost sheen and boundary oil film. This kind of film takes a long time to form. Out goes the sludge and also this valuable surface film during flushing. In an opened engine, this film is like the colour of honey or shellac.