Gunk may work, but I'm not sure how it will impact your seals. I've heard some concerns over using these harsh flushes because of the seals and possible plugging of the pump screen.
The ultimate flush appears to be the Auto-rx method based upon results posted to this forum. You have to add it and run it for 500 miles I think then change the oil again. Since you were planning on doing it twice anyway, this is probably the safer and better way to flush the engine. It cleans it slowly, but safely. With Gunk, it might break loose some crud and plug up your screen on the pump.
Or you could choose to do neither and just switch to a high grade synthetic such as Mobil 1, Redline, or Amsoil to name a few. These tend to have cleaning properties based upon what I've heard.
Oh and I've done the ATF "Flush" and I'm not sure how well that really works. I drained the old oil and used cheap dino oil with ATF and run it for about 10 minutes and drained. Finally I added Mobil 1 with a good filter. The oil is staying pretty clean on my 100,000 mile toyota engine, so maybe it worked. If I were to do it again, I would probably go the Auto-rx route even though it costs a bit more.
[ December 16, 2002, 03:32 PM: Message edited by: Giles ]