I can tell you that in my area of the country we have not seen any issues with the 4.7 as far as major engine failure goes. That being said, we have had issues with different vehicles that other areas don't. Back in the late 90's, we had mold forming inside the fuel tanks of Dodge trucks that caused fuel pumps to fail that the rest of the country never had.
One thing about engine flushes is if you don't do them on a regular basis and you have a sludge problem the flush can loosen up the deposits and plug your pickup screen. So if you have a choice on cleaning the internal engine, remove the valve covers and oil pan and hot tank them to remove the crud that may be built up.
As far as the sludge you see are you speaking of the light brown goo that forms in the oil fill tube? That is a normal by-product of moisture mixing with the oil and this is the highest point of the engine for the moisture to collect. I see it in most every 4.7 I have worked on and does not generally mean the entire engine looks the same way.
We have a hot tank or hot water chemical bath that uses hot water and high pressure spray to help loosen and remove build up. Maybe a local machine shop would do that for you. It does a very good job and you will be pleased with the results.
If by chance you have heavy build up when you get the valve covers and oil pan off you can do a bit of scrapping and use a degreaser to help with getting it out of the block. If it looks to be severe you might want to consider removing the entire block and sending it to the shop for cleaning.
I really cannot verify major issues with this engine like I have seen with the older 2.7 liter engines in the Dodge Intrepids. Those where some serious disasters. I saw more sludge issues than not with these engines but it was mainly due to neglect and poor maintenance.
I am very partial to Castrol and use it in both of the vehicles that I own. I use standard oil and not synthetic because there is no greater longevity in using it over the standard oil especially if it is changed at 3k miles.
I also have a recommendation for an additive. I don't know if you have heard of BG Products but they claim to warranty the engine and anything else you use it in over the standard time. Go to www.bgfindashop.com to get the particulars.