I used it on a few bikes i restored, mostly the chrome parts, haven't tried it yet on engine/car stuff yet. works real nice on chrome, usually between a teaspoon to tablespoon per gallon is all you need for delicate stuff like chrome or items that have paint. You can probably go heavier for cast iron stuff. Here is a link to the musclebike forum where i learned about it. I got mine at DIY/ACE hardware for like $5.
http://musclebikeforums.com/topic/8830/Wood-Bleach-Oxalic-Acid