I agree, you don't want to "flush" the motor. What happens is the accumulated, hardened sludge breaks loose, goes into your oil pan and clogs up your oil pickup. What you want is something that will SLOWLY dissolve the sludge.
Read this site for an explanation:
http://www.auto-rx.com/ I'm not recommending that particular product, as I have not used it. However I would use this before an unknown like ATF or the "5 minute" flushes.
I bought a E250 van with the 300 I6, the motor had no oil pressure. Immediately before the engine lost oil pressure, the owner decided to use 15W40 (high detergent diesel oil) in it. Upon removing the oil pan, I found the oil pickup was COMPLETELY blocked with sludge. The oil had dissolved the sludge in the top end, and it had re-deposited and hardened on the pickup.