There are two kinds of engine oil flushes. One that is done chemically by adding something to the oil, usually just before changing it. The other is with a machine that that uses a special solvent that is circulated through the engine with the engine off. Bilstein makes one of the more popular ones. Most of the machines work very well. Chemical flushes are usually not nearly as effective with the possible exception of Auto RX. There is quite a bit of buzz about Auto RX at
www.bobistheoilguy.com
Back to your original question. It is my opinion that it never needs to be done if you are using a quality oil (syn or dino) and you change it frequently enough.
If the car has had spotty maintenance or ever been overheated it may benefit from such a service.
Also my opinion if quality dino is used and it is changed at least every 5,000 miles or syn is used and it is changed at least every 10,000 miles, you probably will never need a flush.
I view an engine flush as being borderline on being snake oil. There are times that it may be beneficial, but not nearly as often as many shops try to sell it.
