If you do this, I would pull the oil pan after running it for 15 or 20 minutes, and keep an eye on the oil pressure gauge while it is running. It is common to knock crud loose and plug the oil pickup screen when trying to flush-clean an engine. That is why a lot of people will recommend against it. If you are going to rebuild it anyway, fine, but if you think the engine is salvageable without a rebuild I wouldn't do it unless you are prepared to pull the pan and scoop out the crud.
