http://www.carbibles.com/engineoil_bible.html
Flushing oils
These are special compound oils that are very, very thin. They almost have the consistency of tap water when cold as well as hot. Typically they are 0W/20 oils.
Don't ever drive with these oils in the engine - it won't last. (Caveat : some hybrid vehicles now require 0W20, so if you're a hybrid driver, check your owner's manual). Their purpose is for cleaning out all the gunk which builds up inside an engine.
Note that Mobil1 0W40 is okay, because the '40' denotes that it's actually thick enough at temperature to work. 0W20 just doesn't get that viscous! To use them, drain your engine of all it's oil, but leave the old oil filter in place. Next fill it up with flushing oil and run it at a fast idle for about 20 minutes. Finally, drain all this off (and marvel at the crap that comes out with it), replace the oil filter, refill with a good synthetic oil and voila! Clean engine.
Of course, like most things nowadays, there's a condition attached when using flushing oils. In an old engine you really don't want to remove all the deposits. Some of these deposits help seal rings, lifters and even some of the flanges between the heads, covers, pan and the block, where the gaskets are thin. I have heard of engines with over 280,000km that worked fine, but when flushed it failed in a month because the blow-by past the scraper ring(now really clean)contaminated the oil and screwed the rod bearings.
