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RydLyme is a *buffered* hydrdochloric acid. The buffering agent prevents the reduction of the normally formed protective 'black' (ferrous) rust into destructive 'red (ferric) rust ... so that you can descale the carbonate formations ... and with minimal attack of the engine's base metal. Simple speak: RydLyme and other buffered descaling agents remove the 'insoluble salts' but not (much of) the 'metals'. Straight unbuffered HCl will bore through the carbonates, through the 'rust' and into the metal ... and since cast iron is typically a 'stratified' casting (like an onion) once you penetrate one of these 'onion layers' you promote 'slab rust' - large 'plateletts' of ferric rust that can loosen and migrate further into the engine and exhaust manifold and can cause partial or total blocking of the cooling circuit.
Oxalic is 'too weak' an acid to adequately react with the formed carbonates .... the 'reactivity' of oxalic vs. carbonates will require an immense amount of 'oxalic' or 'vinegar' to do the job.