You know those oil flush additives you see on t.v. and at your tires stores? they are CRAP.
Engine oil already has additives added to clean your engine components. Its redundant to add more. Now the negative side affects, oil flush additive is just a solvent. It does not lubricate, it will thin-out your oil, causing more harm then good. If you want to get rid of sludge build up, do more frequent oil changes, or do them on time - 5000km.