Tell me about this please lol
you can use oxalic too... actually you can combine the 2 too... but we tend to just buy muriatic..... it works better on bronze which we see TONS of...
there aren't any real guidelines to acid strengths vs materials per se, occasionally you'll get a manufacturers recommendations on flushing, but it's usually an experienced "know what the metal of said item" can handle, and how long...
that being said... radiators are pretty dainty... i never run the concentration of acid that i run on lots of boat parts on a rad.... usually i'll add a gal of muriatic to about 2 or 3 gallons of water... NOTE: ALWAY ADD THE ACID TO THE WATER, NOT VICE VERSA! and yes, gloves, goggles and a respirator are advised.... take it someone who knows VERY well what it's like to have the oxygen sucked out of your lungs from the acid fumes...

not fun...
also.... thats a pretty mild solution.... i'd generally let that sit in the rad for about 15, 20 minutes... if you go to the local hardware store and look at the muriatic gals, the %'s are VERY low today... usually they only have a couple % acid in em anymore... used to get it and it was 15, maybe %20....
flush very well..... sometimes i'll mix a couple tablespoon of baking soda in a couple gals of water and run that thru, as baking soda nuetralizes the acid... then cleanwater flush....
DISCLAIMER....... I'm taking no responsibility for anyone nuking their rad..... use at own risk... borderline failure parts, can fail.....