I have been watching these "ATF as engine flush" threads for years. I have seen so many different answers, including a page from Amsoil... and most of them are actually WRONG! In the early 90's I was (still am) a Mazda master tech, and a lot of the Mazda engines back then, especially the V6 engine, had lifter noise as early as 20,000 kms. Mazda came out with a TSB and a process to clean these lifters without replacing them using ATF and it worked amazingly well. In the decades since, I have worked for almost every import brand sold in North America and have used ATF to free up stuck rings and internally clean engines in Mazda, Nissan, Honda, Hyundai, Kia, Gm 4.8 and 5.3, the list goes on. ATF has detergents that will break down carbon build up (the detergent then bonds to particulates and gets stuck in the filter), friction modifiers (we don't care about those), seal conditioners which are good for main seals and valve seals, and due to the longer service interval the base oil is usually very good. As far as the weight it depends on brand and whether it is conventional, blend or full synthetic, but it can range from 5 to 10 "usually". I have always had better luck with Dex III or ATF+4. I have found many of the full synthetics don't seem to work as well. "Unsticking" oil control rings has saved MANY of my customers a lot of money over the years and gained my shop a good reputation. I have tried almost every engine cleaner/flush that I have found on the market, I don't really have anything negative to say other than one thing... high rpm, close tolerance engines, with rubber seals... mixed with a solvent based cleaner "in my experience" is not a good thing. It may work, most of the time it may work, but it only takes once on a customers car for it to cause damage and you are on the hook for it. Anybody who has owned their own shop knows how tight the margins are in the auto repair industry
