LOTS of posts here. ANYONE a Chemistry Degree? A LOT of folks have spent a LOT of years studying the principles involved in cleaning/ degreasing, and testing different products. I'm NOT one of those folks, so I won't express an opinion other than "Water won't cut it".
While it is true that for an "UN-contaminated"/ Clean cooling system, you ONLY want to add distilled water and proper Anti-freeze with correct Corrosion Inhibitor, that is NOT OP's case. He has a LOT of Oil-based Sludge in the system. Why do environmentalists clean seabirds that have gotten coated by an oil slick. Nothing better to do? If Water "alone would fix it", just drop them off someplace with NO Oil.
So MAYBE we should actually READ and try to understand what the folks that have spent years learning the principles of chemistry, and specializing in "Surfactants", Emulsifiers, pH, and other related issues have written. You'll wait a LOOOONG time for the Movie, or even the Video.
Generally speaking, it appears that OP's MOST pressing concern is OVER-HEATING, and that he needs to concentrate on (1) improving coolant flow in the Water Jacket, and (2) improving heat transfer in water jacket and the radiator. BOTH of those require getting rid of the "Sludge". If he doesn't do that, car will overheat and damage will be done to major engine parts, OR he just can't drive it.
"Prestone Radiator Flush & Oil Degreaser" looks like a good product to EVALUATE. The SDS (Safety Data Sheet) was published December 2019, so it's been around long enough for any major issues from its use to have been reported. It has TWO basic ingredients:
1) Sodium Citrate, at least 5%; according to Wiki- "Sodium citrates are used as acidity regulators in food and drinks, and also as emulsifiers for oils."
2) Nonionic Surfactant; Wiki- "Surfactants may act as detergents, wetting agents, emulsifiers... Non-ionic surfactants are less sensitive to water hardness than anionic surfactants, and they foam less strongly."
So for anyone with Oil in Coolant, I would suggest reading the following and talking to some Shop Folk who see such a situation regularly:
https://prestone.com/product/preston...oil-degreaser/
https://prestone.com/wp-content/uplo...hDegreaser.pdf
https://en.wikipedia.org/wiki/Sodium_citrate
https://en.wikipedia.org/wiki/Surfactant#Non-ionic
And DO read any Directions/Instructions related to product use. The Temperature of the system, and HOW LONG you leave it in the system are important. There are BOTH (A) Chemical Reactions, and (B) Mechanical Actions involved in any cleaning operation. The Chemical part is what most of us don't understand, and Time & Temperature are important.
George